hscontrol: bound pending authentication without evicting sessions

CHANGELOG.md

@@ -7,6 +7,9 @@
 ### Security
 
 - Metrics are no longer exposed to unregistered clients through the Noise protocol. They remain available only on the configured metrics listener; an empty `metrics_listen_addr` disables that listener. [#3571](https://github.com/juanfont/headscale/issues/3571)
+- Preserve pending registrations and OIDC logins at capacity instead of evicting active approvals; refuse excess work with a retry response and reserve capacity for registered nodes' SSH checks.
+- Retain pending registration data as owned serialized payloads capped at 64 KiB, preserving tags and expiry while bounding stored data. Oversized interactive registrations are rejected explicitly.
+- Validate OIDC login session type before allocating state, consume callback state atomically, and release SSH authentication capacity after the client consumes its verdict.
 
 ### v1 REST API replaced; gRPC and Protobuf removed
 

config-example.yaml

@@ -510,7 +510,10 @@ auto_update:
 #
 # tuning:
 #   # Maximum number of pending registration entries in the auth cache.
-#   # Oldest entries are evicted when the cap is reached.
+#   # At capacity, new registrations retry without displacing pending approvals.
+#   # SSH checks can use 64 additional reserved entries. Each registration
+#   # retains at most 64 KiB of serialized data, so the default registration
+#   # payload budget is 64 MiB plus record overhead.
 #   #
 #   # register_cache_max_entries: 1024
 #

docs/ref/registration.md

@@ -91,6 +91,34 @@ headscale users create <USER>
     listed as "online" in the output of `headscale nodes list`. The "User" column displays `tagged-devices` as the owner
     of the node. See the "Tags" column for the list of assigned tags.
 
+### Pending authentication limits
+
+Headscale keeps admitted authentication requests until they complete or expire.
+When registration capacity is reached, new requests receive HTTP 503 with
+`Retry-After` instead of invalidating someone else's pending approval. Existing
+approvals and pre-authenticated key registration remain available. SSH checks
+have 64 reserved entries beyond the registration allocation, including in the
+OIDC state cache.
+
+The default registration allocation is 1024 entries. The advanced
+`tuning.register_cache_max_entries` setting changes that allocation. Pending
+registration retains only hostname, OS and requested tags from Hostinfo; other
+metadata arrives with the node's first MapRequest. The full cached registration
+must fit within 64 KiB when serialized; larger payloads are rejected without
+truncating authentication metadata. Noise
+registrations return HTTP 200 with `RegisterResponse.Error` so the Tailscale
+client displays the permanent size error. The admin API reports this size error
+as HTTP 413.
+At the default allocation, retained registration payloads use at most 64 MiB
+plus record overhead. This is a cache-storage bound, not a bound on total server
+memory or concurrent request processing.
+
+OIDC login URLs must reference an existing pending authentication of the matching
+type. OIDC callback state is single-use, including when the identity provider
+exchange fails; restart the browser login flow after such an error. Capacity
+limits protect admitted requests but cannot guarantee admission during sustained
+overload.
+
 ### Pre authenticated key
 
 Registration with a pre authenticated key (or auth key) is a non-interactive way to register a new node. The Headscale

hscontrol/api/v1/errors.go

@@ -20,6 +20,13 @@ func mapError(msg string, err error) error {
 	}
 
 	switch {
+	case errors.Is(err, types.ErrRegistrationPayloadTooLarge):
+		return huma.Error413RequestEntityTooLarge(msg, err)
+
+	case errors.Is(err, state.ErrAuthCacheFull),
+		errors.Is(err, state.ErrAuthCacheClosed):
+		return huma.Error503ServiceUnavailable(msg, err)
+
 	case errors.Is(err, gorm.ErrRecordNotFound),
 		errors.Is(err, state.ErrNodeNotFound),
 		errors.Is(err, state.ErrNodeNotInNodeStore),
@@ -43,7 +50,8 @@ func mapError(msg string, err error) error {
 		return huma.Error400BadRequest(msg, err)
 
 	case errors.Is(err, state.ErrNodeKeyInUse),
-		errors.Is(err, state.ErrAmbiguousNodeOwnership):
+		errors.Is(err, state.ErrAmbiguousNodeOwnership),
+		errors.Is(err, state.ErrAuthCacheEntryExists):
 		return huma.Error409Conflict(msg, err)
 
 	default:

hscontrol/api/v1/nodes.go

@@ -563,7 +563,11 @@ func registerNodeAdminOps(api huma.API, b Backend) {
 		}
 
 		authRegReq := types.NewRegisterAuthRequest(regData)
-		b.State.SetAuthCacheEntry(registrationID, authRegReq)
+
+		err = b.State.SetAuthCacheEntry(registrationID, authRegReq)
+		if err != nil {
+			return nil, mapError("starting authentication", err)
+		}
 
 		// Synthetic echo; the real node is created later via the auth path
 		// from the cached registration data.

hscontrol/apiv1_auth_test.go

@@ -37,7 +37,7 @@ func seedAuthRequest(
 			Hostinfo:   &tailcfg.Hostinfo{Hostname: hostname},
 		}
 
-		app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData))
+		require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData)))
 	}
 }
 
@@ -124,7 +124,7 @@ func TestAPIV1AuthApprove(t *testing.T) {
 
 		authID := types.MustAuthID()
 		authReq := types.NewAuthRequest()
-		h.app.state.SetAuthCacheEntry(authID, authReq)
+		require.NoError(t, h.app.state.SetAuthCacheEntry(authID, authReq))
 
 		body := fmt.Appendf(nil, `{"authId":%q}`, authID.String())
 		res := h.callHuma(http.MethodPost, "/api/v1/auth/approve", body)
@@ -159,7 +159,7 @@ func TestAPIV1AuthReject(t *testing.T) {
 
 		authID := types.MustAuthID()
 		authReq := types.NewAuthRequest()
-		h.app.state.SetAuthCacheEntry(authID, authReq)
+		require.NoError(t, h.app.state.SetAuthCacheEntry(authID, authReq))
 
 		body := fmt.Appendf(nil, `{"authId":%q}`, authID.String())
 		res := h.callHuma(http.MethodPost, "/api/v1/auth/reject", body)

hscontrol/apiv1_nodes_test.go

@@ -364,7 +364,7 @@ func TestAPIV1NodeRegister(t *testing.T) {
 				MachineKey: mk.Public(),
 				Hostname:   "registered-node",
 			}
-			app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData))
+			require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData)))
 		}
 
 		assertParityIsolated(t, seed, http.MethodPost,
@@ -453,3 +453,21 @@ func TestAPIV1NodeDebugCreate(t *testing.T) {
 		assert.Equal(t, "0001-01-01T00:00:00Z", got.Node["lastSeen"])
 	})
 }
+
+func TestAPIV1NodeDebugCreateAdmissionFailure(t *testing.T) {
+	app := createTestApp(t, func(cfg *types.Config) {
+		cfg.Tuning.RegisterCacheMaxEntries = 1
+	})
+	defer func() { require.NoError(t, app.state.Close()) }()
+
+	app.state.CreateUserForTest("alice")
+	require.NoError(t, app.state.SetAuthCacheEntry(types.MustAuthID(), types.NewAuthRequest()))
+
+	id := types.MustAuthID()
+	body := []byte(`{"user":"alice","key":"` + id.String() + `","name":"node","routes":[]}`)
+	res := callHandler(newHumaTestHandler(app), http.MethodPost, "/api/v1/debug/node", body)
+	require.Equal(t, http.StatusServiceUnavailable, res.status)
+
+	_, ok := app.state.GetAuthCacheEntry(id)
+	require.False(t, ok)
+}

hscontrol/auth.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/juanfont/headscale/hscontrol/state"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/rs/zerolog/log"
 	"gorm.io/gorm"
@@ -24,6 +25,17 @@ type AuthProvider interface {
 	AuthURL(authID types.AuthID) string
 }
 
+func authAdmissionError(err error) HTTPError {
+	switch {
+	case errors.Is(err, types.ErrRegistrationPayloadTooLarge):
+		return NewHTTPError(http.StatusRequestEntityTooLarge, err.Error(), err)
+	case errors.Is(err, state.ErrAuthCacheFull), errors.Is(err, state.ErrAuthCacheClosed):
+		return NewHTTPError(http.StatusServiceUnavailable, "cannot start authentication; retry later", err)
+	default:
+		return NewHTTPError(http.StatusInternalServerError, "cannot start authentication", err)
+	}
+}
+
 // machineKeyMismatch fails closed when a node looked up by NodeKey was started
 // in a Noise session with a different machine key. Without this anyone holding a
 // target's NodeKey could open a session with a throwaway machine key and act on
@@ -369,6 +381,8 @@ func (h *Headscale) waitForFollowup(
 			}
 		}
 
+		h.state.DeleteAuthCacheEntry(followupReg)
+
 		if verdict.Accept() {
 			if !verdict.Node.Valid() {
 				// registration is expired in the cache, instruct the client to try a new registration
@@ -397,7 +411,11 @@ func (h *Headscale) reqToNewRegisterResponse(
 	authRegReq := types.NewRegisterAuthRequest(regData)
 
 	log.Info().Msgf("new followup node registration using auth id: %s", newAuthID)
-	h.state.SetAuthCacheEntry(newAuthID, authRegReq)
+
+	err = h.state.SetAuthCacheEntry(newAuthID, authRegReq)
+	if err != nil {
+		return nil, authAdmissionError(err)
+	}
 
 	return &tailcfg.RegisterResponse{
 		AuthURL: h.authProvider.RegisterURL(newAuthID),
@@ -532,7 +550,10 @@ func (h *Headscale) handleRegisterInteractive(
 		registrationDataFromRequest(req, machineKey),
 	)
 
-	h.state.SetAuthCacheEntry(authID, authRegReq)
+	err = h.state.SetAuthCacheEntry(authID, authRegReq)
+	if err != nil {
+		return nil, authAdmissionError(err)
+	}
 
 	log.Info().Msgf("starting node registration using auth id: %s", authID)
 

hscontrol/auth_tags_test.go

@@ -847,7 +847,7 @@ func TestExpiryDuringPersonalToTaggedConversion(t *testing.T) {
 		},
 		Expiry: &clientExpiry,
 	})
-	app.state.SetAuthCacheEntry(registrationID1, regEntry1)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID1, regEntry1))
 
 	node, _, err := app.state.HandleNodeFromAuthPath(
 		registrationID1, types.UserID(user.ID), nil, "webauth",
@@ -869,7 +869,7 @@ func TestExpiryDuringPersonalToTaggedConversion(t *testing.T) {
 		},
 		Expiry: &clientExpiry, // Client still sends expiry
 	})
-	app.state.SetAuthCacheEntry(registrationID2, regEntry2)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID2, regEntry2))
 
 	nodeAfter, _, err := app.state.HandleNodeFromAuthPath(
 		registrationID2, types.UserID(user.ID), nil, "webauth",
@@ -918,7 +918,7 @@ func TestExpiryDuringTaggedToPersonalConversion(t *testing.T) {
 			RequestTags: []string{"tag:server"}, // Tagged node
 		},
 	})
-	app.state.SetAuthCacheEntry(registrationID1, regEntry1)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID1, regEntry1))
 
 	node, _, err := app.state.HandleNodeFromAuthPath(
 		registrationID1, types.UserID(user.ID), nil, "webauth",
@@ -941,7 +941,7 @@ func TestExpiryDuringTaggedToPersonalConversion(t *testing.T) {
 		},
 		Expiry: &clientExpiry, // Client requests expiry
 	})
-	app.state.SetAuthCacheEntry(registrationID2, regEntry2)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID2, regEntry2))
 
 	nodeAfter, _, err := app.state.HandleNodeFromAuthPath(
 		registrationID2, types.UserID(user.ID), nil, "webauth",

hscontrol/auth_test.go

@@ -13,7 +13,9 @@ import (
 	"time"
 
 	"github.com/juanfont/headscale/hscontrol/mapper"
+	"github.com/juanfont/headscale/hscontrol/state"
 	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/rs/zerolog"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"tailscale.com/tailcfg"
@@ -689,7 +691,7 @@ func TestAuthenticationFlows(t *testing.T) {
 					MachineKey: machineKey1.Public(),
 					Hostname:   "followup-success-node",
 				})
-				app.state.SetAuthCacheEntry(regID, nodeToRegister)
+				require.NoError(t, app.state.SetAuthCacheEntry(regID, nodeToRegister))
 
 				// Simulate successful registration
 				// [Headscale.handleRegister] will receive the value when it starts waiting
@@ -737,7 +739,7 @@ func TestAuthenticationFlows(t *testing.T) {
 					MachineKey: machineKey1.Public(),
 					Hostname:   "followup-timeout-node",
 				})
-				app.state.SetAuthCacheEntry(regID, nodeToRegister)
+				require.NoError(t, app.state.SetAuthCacheEntry(regID, nodeToRegister))
 				// Don't call FinishRegistration - will timeout
 
 				return fmt.Sprintf("http://localhost:8080/register/%s", regID), nil
@@ -1575,7 +1577,7 @@ func TestAuthenticationFlows(t *testing.T) {
 					MachineKey: machineKey1.Public(),
 					Hostname:   "nil-response-node",
 				})
-				app.state.SetAuthCacheEntry(regID, nodeToRegister)
+				require.NoError(t, app.state.SetAuthCacheEntry(regID, nodeToRegister))
 
 				// Simulate registration that returns empty NodeView (cache expired during auth)
 				go func() {
@@ -3356,7 +3358,7 @@ func TestWebFlowReauthDifferentUser(t *testing.T) {
 }
 
 // Helper function to create test app.
-func createTestApp(t *testing.T) *Headscale {
+func createTestApp(t *testing.T, options ...func(*types.Config)) *Headscale {
 	t.Helper()
 
 	tmpDir := t.TempDir()
@@ -3380,6 +3382,10 @@ func createTestApp(t *testing.T) *Headscale {
 		},
 	}
 
+	for _, option := range options {
+		option(&cfg)
+	}
+
 	app, err := NewHeadscale(&cfg)
 	require.NoError(t, err)
 
@@ -3397,6 +3403,106 @@ func createTestApp(t *testing.T) *Headscale {
 	return app
 }
 
+func TestRegistrationFollowupReleasesRejectedSession(t *testing.T) {
+	app := createTestApp(t, func(cfg *types.Config) {
+		cfg.Tuning.RegisterCacheMaxEntries = 1
+	})
+	defer func() { require.NoError(t, app.state.Close()) }()
+
+	machineKey := key.NewMachine().Public()
+	req := tailcfg.RegisterRequest{
+		NodeKey:  key.NewNode().Public(),
+		Hostinfo: &tailcfg.Hostinfo{Hostname: "rejected-registration"},
+	}
+	initial, err := app.handleRegisterInteractive(req, machineKey)
+	require.NoError(t, err)
+
+	req.Followup = initial.AuthURL
+	initialURL, err := url.Parse(initial.AuthURL)
+	require.NoError(t, err)
+	authID, err := types.AuthIDFromString(strings.TrimPrefix(initialURL.Path, "/register/"))
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	_, err = app.waitForFollowup(ctx, req.Followup, registrationDataFromRequest(req, machineKey))
+	require.ErrorIs(t, err, context.Canceled)
+
+	_, present := app.state.GetAuthCacheEntry(authID)
+	require.True(t, present, "a canceled poll must leave the pending session available")
+
+	body := fmt.Appendf(nil, `{"authId":%q}`, authID.String())
+	rejected := callHandler(newHumaTestHandler(app), http.MethodPost, "/api/v1/auth/reject", body)
+	require.Equal(t, http.StatusOK, rejected.status)
+
+	_, present = app.state.GetAuthCacheEntry(authID)
+	require.True(t, present, "the verdict must remain available until follow-up consumes it")
+
+	followup, err := app.waitForFollowup(t.Context(), req.Followup, registrationDataFromRequest(req, machineKey))
+	require.NoError(t, err, "consuming the rejection must free capacity for the replacement")
+	require.NotEmpty(t, followup.AuthURL)
+	require.NotEqual(t, initial.AuthURL, followup.AuthURL)
+
+	_, present = app.state.GetAuthCacheEntry(authID)
+	require.False(t, present, "the consumed session must release its slot")
+}
+
+func TestAuthAdmissionErrors(t *testing.T) {
+	for _, flow := range []string{"interactive", "followup", "ssh"} {
+		t.Run(flow, func(t *testing.T) {
+			app := createTestApp(t, func(cfg *types.Config) {
+				cfg.Tuning.RegisterCacheMaxEntries = 1
+			})
+			defer func() { require.NoError(t, app.state.Close()) }()
+
+			id := types.MustAuthID()
+			pending := types.NewAuthRequest()
+			require.NoError(t, app.state.SetAuthCacheEntry(id, pending))
+
+			if flow == "ssh" {
+				for range 64 {
+					require.NoError(t, app.state.SetAuthCacheEntry(types.MustAuthID(), types.NewSSHCheckAuthRequest(1, 2)))
+				}
+			}
+
+			req := tailcfg.RegisterRequest{
+				NodeKey:  key.NewNode().Public(),
+				Hostinfo: &tailcfg.Hostinfo{Hostname: "pending-node"},
+			}
+			machineKey := key.NewMachine().Public()
+
+			var err error
+
+			switch flow {
+			case "interactive":
+				var resp *tailcfg.RegisterResponse
+
+				resp, err = app.handleRegisterInteractive(req, machineKey)
+				require.Nil(t, resp, "refused admission must not publish an auth URL")
+			case "followup":
+				var resp *tailcfg.RegisterResponse
+
+				resp, err = app.reqToNewRegisterResponse(registrationDataFromRequest(req, machineKey))
+				require.Nil(t, resp, "refused admission must not publish an auth URL")
+			case "ssh":
+				ns := &noiseServer{headscale: app}
+
+				var action *tailcfg.SSHAction
+
+				action, err = ns.sshActionHoldAndDelegate(zerolog.Nop(), &tailcfg.SSHAction{}, 1, 2, "alice")
+				require.Nil(t, action, "refused admission must not publish a delegation URL")
+			}
+
+			require.ErrorIs(t, err, state.ErrAuthCacheFull)
+
+			got, ok := app.state.GetAuthCacheEntry(id)
+			require.True(t, ok)
+			require.Same(t, pending, got)
+		})
+	}
+}
+
 // TestGitHubIssue2830_NodeRestartWithUsedPreAuthKey tests the scenario reported in
 // https://github.com/juanfont/headscale/issues/2830
 //
@@ -3819,7 +3925,7 @@ func TestWebAuthRejectsUnauthorizedRequestTags(t *testing.T) {
 			RequestTags: []string{"tag:unauthorized"}, // This tag is not in policy
 		},
 	})
-	app.state.SetAuthCacheEntry(registrationID, regEntry)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID, regEntry))
 
 	// Complete the web auth - should fail because tag is unauthorized
 	_, _, err := app.state.HandleNodeFromAuthPath(
@@ -3882,7 +3988,7 @@ func TestWebAuthReauthWithEmptyTagsRemovesAllTags(t *testing.T) {
 			RequestTags: []string{"tag:valid-owned", "tag:second"},
 		},
 	})
-	app.state.SetAuthCacheEntry(registrationID1, regEntry1)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID1, regEntry1))
 
 	// Complete initial registration with tags
 	node, _, err := app.state.HandleNodeFromAuthPath(
@@ -3909,7 +4015,7 @@ func TestWebAuthReauthWithEmptyTagsRemovesAllTags(t *testing.T) {
 			RequestTags: []string{}, // EMPTY - should untag
 		},
 	})
-	app.state.SetAuthCacheEntry(registrationID2, regEntry2)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID2, regEntry2))
 
 	// Complete reauth with empty tags
 	nodeAfterReauth, _, err := app.state.HandleNodeFromAuthPath(
@@ -3995,7 +4101,7 @@ func TestAuthKeyTaggedToUserOwnedViaReauth(t *testing.T) {
 			RequestTags: []string{}, // EMPTY - should untag
 		},
 	})
-	app.state.SetAuthCacheEntry(registrationID, regEntry)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID, regEntry))
 
 	// Complete reauth with empty tags
 	nodeAfterReauth, _, err := app.state.HandleNodeFromAuthPath(
@@ -4195,7 +4301,7 @@ func TestTaggedNodeWithoutUserToDifferentUser(t *testing.T) {
 			RequestTags: []string{}, // Empty - transition to user-owned
 		},
 	})
-	app.state.SetAuthCacheEntry(registrationID, regEntry)
+	require.NoError(t, app.state.SetAuthCacheEntry(registrationID, regEntry))
 
 	// This should NOT panic - before the fix, this would panic with:
 	// panic: runtime error: invalid memory address or nil pointer dereference
@@ -4329,7 +4435,7 @@ func TestHandleNodeFromAuthPath_OldUserNil_NoPanic(t *testing.T) {
 			Hostname: "authpath-orphan-newuser",
 		},
 	})
-	app.state.SetAuthCacheEntry(authID, regEntry)
+	require.NoError(t, app.state.SetAuthCacheEntry(authID, regEntry))
 
 	node, _, err := app.state.HandleNodeFromAuthPath(
 		authID,
@@ -4384,14 +4490,14 @@ func TestHandleNodeFromAuthPath_ReloginWholeNode(t *testing.T) {
 
 			newNodeKey := key.NewNode()
 			authID := types.MustAuthID()
-			app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+			require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 				MachineKey: node.MachineKey,
 				NodeKey:    newNodeKey.Public(),
 				Hostname:   hostname,
 				Hostinfo: &tailcfg.Hostinfo{
 					Hostname: hostname,
 				},
-			}))
+			})))
 
 			relogged, c, err := app.state.HandleNodeFromAuthPath(
 				authID,
@@ -4443,7 +4549,7 @@ func TestWaitForFollowupMachineKeyMismatch(t *testing.T) {
 			NodeKey:    key.NewNode().Public(),
 			Hostname:   hostname,
 		})
-		app.state.SetAuthCacheEntry(authID, regEntry)
+		require.NoError(t, app.state.SetAuthCacheEntry(authID, regEntry))
 
 		user := app.state.CreateUserForTest(hostname + "-user")
 		node := app.state.CreateNodeForTest(user, hostname)
@@ -4520,7 +4626,7 @@ func TestWaitForFollowupMachineKeyMismatch(t *testing.T) {
 			NodeKey:    key.NewNode().Public(),
 			Hostname:   "followup-pending",
 		})
-		app.state.SetAuthCacheEntry(authID, regEntry)
+		require.NoError(t, app.state.SetAuthCacheEntry(authID, regEntry))
 
 		url := fmt.Sprintf("http://localhost:8080/register/%s", authID)
 		refusedAtOnce(t, url, foreignMachineKey.Public())
@@ -4539,7 +4645,7 @@ func TestWaitForFollowupMachineKeyMismatch(t *testing.T) {
 
 	t.Run("ssh check session is not a registration", func(t *testing.T) {
 		authID := types.MustAuthID()
-		app.state.SetAuthCacheEntry(authID, types.NewSSHCheckAuthRequest(1, 2))
+		require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewSSHCheckAuthRequest(1, 2)))
 
 		refusedAtOnce(t, fmt.Sprintf("http://localhost:8080/register/%s", authID),
 			victimMachineKey.Public())
@@ -4583,7 +4689,7 @@ func TestFollowupWaitPrefersCompletedAuthOverExpiredContext(t *testing.T) {
 			MachineKey: machineKey,
 			Hostname:   "followup-race-node",
 		})
-		app.state.SetAuthCacheEntry(regID, authReq)
+		require.NoError(t, app.state.SetAuthCacheEntry(regID, authReq))
 
 		// Registration completes BEFORE we wait: verdict is buffered.
 		user := app.state.CreateUserForTest(fmt.Sprintf("followup-race-user-%d", i))
@@ -4711,7 +4817,7 @@ func TestFollowupParksOneRequestPerRegistration(t *testing.T) {
 		MachineKey: machineKey,
 		Hostname:   "followup-park-node",
 	})
-	app.state.SetAuthCacheEntry(authID, reg)
+	require.NoError(t, app.state.SetAuthCacheEntry(authID, reg))
 
 	regData := &types.RegistrationData{
 		MachineKey: machineKey,
@@ -4833,7 +4939,7 @@ func TestFollowupRefreshKeepsAuthMetadata(t *testing.T) {
 				reg := types.NewRegisterAuthRequest(&types.RegistrationData{
 					MachineKey: machineKey, Hostname: "previous-request",
 				})
-				app.state.SetAuthCacheEntry(authID, reg)
+				require.NoError(t, app.state.SetAuthCacheEntry(authID, reg))
 				reg.FinishAuth(types.AuthVerdict{})
 			}
 

hscontrol/noise.go

@@ -566,10 +566,13 @@ func (ns *noiseServer) sshActionHoldAndDelegate(
 		)
 	}
 
-	ns.headscale.state.SetAuthCacheEntry(
+	err = ns.headscale.state.SetAuthCacheEntry(
 		authID,
 		types.NewSSHCheckAuthRequest(srcNodeID, dstNodeID),
 	)
+	if err != nil {
+		return nil, authAdmissionError(err)
+	}
 
 	authURL := ns.headscale.authProvider.AuthURL(authID)
 
@@ -691,6 +694,8 @@ func (ns *noiseServer) sshActionFollowUp(
 	case verdict, verdictOK = <-auth.WaitForAuth():
 	}
 
+	ns.headscale.state.DeleteAuthCacheEntry(authID)
+
 	// FinishAuth buffers one verdict, then closes the channel. A closed
 	// receive yields the zero verdict, which Accept() reports as success,
 	// so a replayed follow-up would be approved even after a Reject.
@@ -1004,6 +1009,7 @@ func (ns *noiseServer) RegistrationHandler(
 		return
 	}
 
+	statusCode := http.StatusOK
 	registerResponse := func() *tailcfg.RegisterResponse { //nolint:contextcheck
 		if decodeErr != nil {
 			return regErr(decodeErr)
@@ -1015,6 +1021,13 @@ func (ns *noiseServer) RegistrationHandler(
 		resp, err := ns.headscale.handleRegister(req.Context(), registerRequest, ns.machineKey)
 		if err != nil {
 			if httpErr, ok := errors.AsType[HTTPError](err); ok {
+				// Keep permanent errors in HTTP 200 so controlclient surfaces RegisterResponse.Error.
+				if httpErr.Code == http.StatusServiceUnavailable {
+					statusCode = http.StatusServiceUnavailable
+
+					writer.Header().Set("Retry-After", "10")
+				}
+
 				return &tailcfg.RegisterResponse{
 					Error: httpErr.Msg,
 				}
@@ -1027,7 +1040,7 @@ func (ns *noiseServer) RegistrationHandler(
 	}()
 
 	writer.Header().Set("Content-Type", "application/json; charset=utf-8")
-	writer.WriteHeader(http.StatusOK)
+	writer.WriteHeader(statusCode)
 
 	err := json.NewEncoder(writer).Encode(registerResponse)
 	if err != nil {

hscontrol/noise_test.go

@@ -358,6 +358,69 @@ func serveRegister(t *testing.T, ns *noiseServer, body any) *httptest.ResponseRe
 	return rec
 }
 
+func TestRegistrationHandler_AdmissionRetry(t *testing.T) {
+	app := createTestApp(t, func(cfg *types.Config) {
+		cfg.Tuning.RegisterCacheMaxEntries = 1
+	})
+	defer func() { require.NoError(t, app.state.Close()) }()
+
+	require.NoError(t, app.state.SetAuthCacheEntry(types.MustAuthID(), types.NewAuthRequest()))
+
+	ns := &noiseServer{headscale: app, machineKey: key.NewMachine().Public()}
+	rec := serveRegister(t, ns, tailcfg.RegisterRequest{
+		Version:  capver.MinSupportedCapabilityVersion,
+		NodeKey:  key.NewNode().Public(),
+		Hostinfo: &tailcfg.Hostinfo{Hostname: "node"},
+	})
+	require.Equal(t, http.StatusServiceUnavailable, rec.Code)
+	require.NotEmpty(t, rec.Header().Get("Retry-After"))
+
+	var response tailcfg.RegisterResponse
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &response))
+	require.Empty(t, response.AuthURL)
+	require.NotEmpty(t, response.Error)
+}
+
+func TestRegistrationHandler_RegistrationPayloadLimit(t *testing.T) {
+	app := createTestApp(t)
+	defer func() { require.NoError(t, app.state.Close()) }()
+
+	hostinfo := &tailcfg.Hostinfo{Hostname: "node", OS: strings.Repeat("x", types.MaxHostinfoSize-128)}
+	require.NoError(t, types.ValidateHostinfo(hostinfo), "the full registration must reach cache admission")
+
+	ns := &noiseServer{headscale: app, machineKey: key.NewMachine().Public()}
+	rec := serveRegister(t, ns, tailcfg.RegisterRequest{
+		Version:  capver.MinSupportedCapabilityVersion,
+		NodeKey:  key.NewNode().Public(),
+		Hostinfo: hostinfo,
+	})
+	require.Equal(t, http.StatusOK, rec.Code, "controlclient only surfaces RegisterResponse.Error with HTTP 200")
+	require.Empty(t, rec.Header().Get("Retry-After"))
+
+	var response tailcfg.RegisterResponse
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &response))
+	require.Contains(t, response.Error, types.ErrRegistrationPayloadTooLarge.Error())
+	require.Contains(t, response.Error, strconv.Itoa(types.MaxRegistrationPayloadBytes))
+	require.Empty(t, response.AuthURL)
+	require.False(t, response.MachineAuthorized)
+	require.Zero(t, app.state.DebugRegistrationCache()["current_len"], "oversized registrations must not enter the auth cache")
+}
+
+func TestSSHActionFollowUp_ReleasesConsumedSession(t *testing.T) {
+	f := newSSHVerdictFixture(t)
+	authID, auth := f.mint(t)
+	auth.FinishAuth(types.AuthVerdict{})
+
+	_, ok := f.ns.headscale.state.GetAuthCacheEntry(authID)
+	require.True(t, ok, "approval must remain available until the client consumes it")
+
+	action := sshActionFromRecorder(t, f.followUp(t, authID))
+	require.True(t, action.Accept)
+
+	_, ok = f.ns.headscale.state.GetAuthCacheEntry(authID)
+	require.False(t, ok, "a consumed verdict must release its admission slot")
+}
+
 func requireBelowFloorRejected(t *testing.T, rec *httptest.ResponseRecorder) {
 	t.Helper()
 
@@ -721,10 +784,10 @@ func TestSSHActionFollowUp_RejectsBindingMismatch(t *testing.T) {
 
 	// Mint an SSH-check auth request bound to (srcCached, dstCached).
 	authID := types.MustAuthID()
-	app.state.SetAuthCacheEntry(
+	require.NoError(t, app.state.SetAuthCacheEntry(
 		authID,
 		types.NewSSHCheckAuthRequest(srcCached.ID, dstCached.ID),
-	)
+	))
 
 	// Build a follow-up that claims to be for (srcOther, dstOther) but
 	// reuses the bound auth_id. The Noise machineKey matches dstOther so
@@ -881,7 +944,7 @@ func TestSSHActionFollowUp_RejectsVerdictAfterCheckRemoved(t *testing.T) {
 			require.NoError(t, err)
 
 			authID := types.MustAuthID()
-			app.state.SetAuthCacheEntry(authID, types.NewSSHCheckAuthRequest(src.ID, dst.ID))
+			require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewSSHCheckAuthRequest(src.ID, dst.ID)))
 
 			auth, ok := app.state.GetAuthCacheEntry(authID)
 			require.True(t, ok)
@@ -2261,7 +2324,7 @@ func TestSSHActionFollowUp_ParksOneRequestPerSession(t *testing.T) {
 
 	authID := types.MustAuthID()
 	auth := types.NewSSHCheckAuthRequest(src.ID, dst.ID)
-	app.state.SetAuthCacheEntry(authID, auth)
+	require.NoError(t, app.state.SetAuthCacheEntry(authID, auth))
 
 	ns := &noiseServer{headscale: app, machineKey: dst.MachineKey}
 

hscontrol/oidc.go

@@ -10,6 +10,8 @@ import (
 	"net/url"
 	"slices"
 	"strings"
+	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/coreos/go-oidc/v3/oidc"
@@ -29,10 +31,10 @@ const (
 	defaultOAuthOptionsCount = 3
 	authCacheExpiration      = time.Minute * 15
 
-	// authCacheMaxEntries bounds the OIDC state→[AuthInfo] cache to prevent
-	// unauthenticated cache-fill DoS via repeated /register/{auth_id} or
-	// /auth/{auth_id} GETs that mint OIDC state cookies.
+	// Registrations may use authCacheMaxEntries slots; SSH checks may also use
+	// the reserve so new registrations cannot fill all pending OIDC capacity.
 	authCacheMaxEntries = 1024
+	authCacheSSHReserve = 64
 
 	// cookieNamePrefixLen is the number of leading characters from a
 	// state/nonce value that [getCookieName] splices into the cookie name.
@@ -47,7 +49,6 @@ var errOIDCStateTooShort = errors.New("oidc state parameter is too short")
 var (
 	errEmptyOIDCCallbackParams = errors.New("empty OIDC callback params")
 	errNoOIDCIDToken           = errors.New("extracting ID token")
-	errNoOIDCRegistrationInfo  = errors.New("registration info not in cache")
 	errOIDCAllowedDomains      = errors.New(
 		"authenticated principal does not match any allowed domain",
 	)
@@ -71,10 +72,11 @@ type AuthProviderOIDC struct {
 	serverURL string
 	cfg       *types.OIDCConfig
 
-	// authCache holds auth information between the auth and the callback
-	// steps. It is a bounded [expirable.LRU] keyed by OIDC state, evicting oldest
-	// entries to keep the cache footprint constant under attack.
-	authCache *expirable.LRU[string, AuthInfo]
+	// authCacheMu serializes non-evicting admission and single-use consumption.
+	// Reads use Peek to preserve insertion order, so the oldest entry expires first.
+	authCacheMu            sync.Mutex
+	authCacheRegistrations atomic.Int64
+	authCache              *expirable.LRU[string, AuthInfo]
 
 	oidcProvider *oidc.Provider
 	oauth2Config *oauth2.Config
@@ -101,21 +103,25 @@ func NewAuthProviderOIDC(
 		Scopes:       cfg.Scope,
 	}
 
-	authCache := expirable.NewLRU[string, AuthInfo](
-		authCacheMaxEntries,
-		nil,
-		authCacheExpiration,
-	)
-
-	return &AuthProviderOIDC{
+	a := &AuthProviderOIDC{
 		h:         h,
 		serverURL: serverURL,
 		cfg:       cfg,
-		authCache: authCache,
 
 		oidcProvider: oidcProvider,
 		oauth2Config: oauth2Config,
-	}, nil
+	}
+	a.authCache = expirable.NewLRU[string, AuthInfo](
+		authCacheMaxEntries+authCacheSSHReserve,
+		func(_ string, info AuthInfo) {
+			if info.Registration {
+				a.authCacheRegistrations.Add(-1)
+			}
+		},
+		authCacheExpiration,
+	)
+
+	return a, nil
 }
 
 // cookiesSecure reports whether the OIDC cookies should carry the Secure flag.
@@ -177,11 +183,24 @@ func (a *AuthProviderOIDC) authHandler(
 		return
 	}
 
-	// Set the state and nonce cookies to protect against CSRF attacks
-	state := a.setCSRFCookie(writer, req, "state")
+	authReq, ok := a.h.state.GetAuthCacheEntry(authID)
+	if !ok {
+		httpUserError(writer, NewHTTPError(http.StatusGone, "login session expired, try again", nil))
+		return
+	}
 
-	// Set the state and nonce cookies to protect against CSRF attacks
-	nonce := a.setCSRFCookie(writer, req, "nonce")
+	if registration && !authReq.IsRegistration() {
+		httpUserError(writer, NewHTTPError(http.StatusBadRequest, "auth session is not for node registration", nil))
+		return
+	}
+
+	if !registration && !authReq.IsSSHCheck() {
+		httpUserError(writer, NewHTTPError(http.StatusBadRequest, "auth session is not for SSH check", nil))
+		return
+	}
+
+	state := rands.HexString(64)
+	nonce := rands.HexString(64)
 
 	registrationInfo := AuthInfo{
 		AuthID:       authID,
@@ -218,8 +237,15 @@ func (a *AuthProviderOIDC) authHandler(
 
 	extras = append(extras, oidc.Nonce(nonce))
 
-	// Cache the registration info
-	a.authCache.Add(state, registrationInfo)
+	if !a.addAuthInfoToState(state, registrationInfo) {
+		writer.Header().Set("Retry-After", "10")
+		httpUserError(writer, NewHTTPError(http.StatusServiceUnavailable, "too many pending OIDC logins, try again later", nil))
+
+		return
+	}
+
+	a.setCSRFCookie(writer, req, "state", state)
+	a.setCSRFCookie(writer, req, "nonce", nonce)
 
 	authURL := a.oauth2Config.AuthCodeURL(state, extras...)
 	log.Debug().Caller().Msgf("redirecting to %s for authentication", authURL)
@@ -255,7 +281,20 @@ func (a *AuthProviderOIDC) OIDCCallbackHandler(
 		return
 	}
 
-	oauth2Token, err := a.getOauth2Token(req.Context(), code, state)
+	// A cookie-bound callback spends its state even if authentication fails;
+	// retrying then requires a fresh login flow rather than reusing the callback.
+	authInfo := a.getAuthInfoFromState(state)
+	if authInfo == nil {
+		httpUserError(writer, NewHTTPError(http.StatusGone, "login session expired, try again", nil))
+		return
+	}
+
+	if req.URL.Query().Get("error") != "" {
+		httpUserError(writer, NewHTTPError(http.StatusForbidden, "OIDC authentication failed", nil))
+		return
+	}
+
+	oauth2Token, err := a.getOauth2Token(req.Context(), code, authInfo)
 	if err != nil {
 		httpUserError(writer, err)
 		return
@@ -343,18 +382,6 @@ func (a *AuthProviderOIDC) OIDCCallbackHandler(
 		return
 	}
 
-	// TODO(kradalby): Is this comment right?
-	// If the node exists, then the node should be reauthenticated,
-	// if the node does not exist, and the machine key exists, then
-	// this is a new node that should be registered.
-	authInfo := a.getAuthInfoFromState(state)
-	if authInfo == nil {
-		log.Debug().Caller().Str("state", state).Msg("state not found in cache, login session may have expired")
-		httpUserError(writer, NewHTTPError(http.StatusGone, "login session expired, try again", nil))
-
-		return
-	}
-
 	// If this is a registration flow, render the confirmation
 	// interstitial instead of finalising the registration immediately.
 	// Without an explicit user click, a single GET to
@@ -471,10 +498,11 @@ func (a *AuthProviderOIDC) determineNodeExpiry(idTokenExpiration time.Time) *tim
 func extractCodeAndStateParamFromRequest(
 	req *http.Request,
 ) (string, string, error) {
-	code := req.URL.Query().Get("code")
-	state := req.URL.Query().Get("state")
+	query := req.URL.Query()
+	code := query.Get("code")
+	state := query.Get("state")
 
-	if code == "" || state == "" {
+	if (code == "" && query.Get("error") == "") || state == "" {
 		return "", "", NewHTTPError(http.StatusBadRequest, "missing code or state parameter", errEmptyOIDCCallbackParams)
 	}
 
@@ -493,19 +521,12 @@ func extractCodeAndStateParamFromRequest(
 func (a *AuthProviderOIDC) getOauth2Token(
 	ctx context.Context,
 	code string,
-	state string,
+	authInfo *AuthInfo,
 ) (*oauth2.Token, error) {
 	var exchangeOpts []oauth2.AuthCodeOption
 
-	if a.cfg.PKCE.Enabled {
-		regInfo, ok := a.authCache.Get(state)
-		if !ok {
-			return nil, NewHTTPError(http.StatusNotFound, "registration not found", errNoOIDCRegistrationInfo)
-		}
-
-		if regInfo.Verifier != nil {
-			exchangeOpts = []oauth2.AuthCodeOption{oauth2.VerifierOption(*regInfo.Verifier)}
-		}
+	if a.cfg.PKCE.Enabled && authInfo.Verifier != nil {
+		exchangeOpts = []oauth2.AuthCodeOption{oauth2.VerifierOption(*authInfo.Verifier)}
 	}
 
 	oauth2Token, err := a.oauth2Config.Exchange(ctx, code, exchangeOpts...)
@@ -631,17 +652,55 @@ func doOIDCAuthorization(
 	return nil
 }
 
+func (a *AuthProviderOIDC) addAuthInfoToState(state string, info AuthInfo) bool {
+	a.authCacheMu.Lock()
+	defer a.authCacheMu.Unlock()
+
+	// Len includes expired entries until the sweeper removes their bucket.
+	for {
+		oldest, _, ok := a.authCache.GetOldest()
+		if !ok {
+			break
+		}
+
+		if _, live := a.authCache.Peek(oldest); live {
+			break
+		}
+
+		a.authCache.Remove(oldest)
+	}
+
+	if a.authCache.Contains(state) || a.authCache.Len() >= a.authCache.Cap() {
+		return false
+	}
+
+	if info.Registration && a.authCacheRegistrations.Load() >= int64(a.authCache.Cap()-authCacheSSHReserve) {
+		return false
+	}
+
+	// Increment before Add: an expiry callback may run as soon as it is stored.
+	if info.Registration {
+		a.authCacheRegistrations.Add(1)
+	}
+
+	a.authCache.Add(state, info)
+
+	return true
+}
+
 // getAuthInfoFromState retrieves and consumes the auth info for a state. The
 // entry is removed on read so a state is single-use: a replayed callback cannot
 // resolve the same auth session twice, even within the cache TTL.
 func (a *AuthProviderOIDC) getAuthInfoFromState(state string) *AuthInfo {
-	authInfo, ok := a.authCache.Get(state)
+	a.authCacheMu.Lock()
+	defer a.authCacheMu.Unlock()
+
+	authInfo, ok := a.authCache.Peek(state)
+	a.authCache.Remove(state)
 	if !ok {
 		return nil
 	}
 
-	a.authCache.Remove(state)
-
 	return &authInfo
 }
 
@@ -1101,9 +1160,8 @@ func (a *AuthProviderOIDC) setCSRFCookie(
 	w http.ResponseWriter,
 	r *http.Request,
 	name string,
-) string {
-	val := rands.HexString(64)
-
+	val string,
+) {
 	//nolint:gosec // G124: Secure from server_url scheme or req.TLS; HttpOnly + SameSite set below
 	c := &http.Cookie{
 		Path:     a.oidcCallbackPath(),
@@ -1120,6 +1178,4 @@ func (a *AuthProviderOIDC) setCSRFCookie(
 		SameSite: http.SameSiteLaxMode,
 	}
 	http.SetCookie(w, c)
-
-	return val
 }

hscontrol/oidc_confirm_test.go

@@ -57,7 +57,7 @@ func TestRegisterConfirmHandler_RejectsCSRFMismatch(t *testing.T) {
 		UserID: 1,
 		CSRF:   "expected-csrf",
 	})
-	app.state.SetAuthCacheEntry(authID, regReq)
+	require.NoError(t, app.state.SetAuthCacheEntry(authID, regReq))
 
 	rec := httptest.NewRecorder()
 	provider.RegisterConfirmHandler(rec,
@@ -89,9 +89,9 @@ func TestRegisterConfirmHandler_RejectsWithoutPending(t *testing.T) {
 	authID := types.MustAuthID()
 	// Cached registration with NO pending confirmation set — i.e. the
 	// OIDC callback has not run yet.
-	app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(
+	require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(
 		&types.RegistrationData{Hostname: "no-oidc-yet"},
-	))
+	)))
 
 	rec := httptest.NewRecorder()
 	provider.RegisterConfirmHandler(rec,

hscontrol/oidc_test.go

@@ -3,6 +3,7 @@ package hscontrol
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
 	"net/http"
 	"net/http/cookiejar"
@@ -10,14 +11,19 @@ import (
 	"net/url"
 	"regexp"
 	"strings"
+	"sync"
+	"sync/atomic"
 	"testing"
+	"testing/synctest"
 	"time"
 
+	"github.com/go-chi/chi/v5"
 	"github.com/hashicorp/golang-lru/v2/expirable"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/oauth2-proxy/mockoidc"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"golang.org/x/oauth2"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 )
@@ -199,7 +205,7 @@ func TestSetCSRFCookieSameSite(t *testing.T) {
 	w := httptest.NewRecorder()
 	r := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/auth/abcdef0123456789", nil)
 
-	a.setCSRFCookie(w, r, "state")
+	a.setCSRFCookie(w, r, "state", "state-value")
 
 	cookies := w.Result().Cookies()
 	require.Len(t, cookies, 1)
@@ -208,7 +214,7 @@ func TestSetCSRFCookieSameSite(t *testing.T) {
 }
 
 // TestExtractCodeAndStateParam covers the callback's first trust-boundary
-// checks: both params required, and a too-short state is rejected before
+// checks: a code or provider error is required, and a too-short state is rejected before
 // getCookieName can slice out of range.
 func TestExtractCodeAndStateParam(t *testing.T) {
 	_, _, err := extractCodeAndStateParamFromRequest(
@@ -242,6 +248,755 @@ func TestGetAuthInfoFromStateSingleUse(t *testing.T) {
 	assert.Nil(t, a.getAuthInfoFromState("state-x"), "a consumed state must not resolve again")
 }
 
+func newOIDCPendingProvider(t *testing.T, capacity int) *AuthProviderOIDC {
+	t.Helper()
+
+	a := &AuthProviderOIDC{
+		h:         createTestApp(t),
+		serverURL: "https://headscale.example",
+		cfg:       &types.OIDCConfig{},
+		oauth2Config: &oauth2.Config{
+			ClientID: "headscale-test",
+			Endpoint: oauth2.Endpoint{
+				AuthURL:   "https://issuer.example/authorize",
+				AuthStyle: oauth2.AuthStyleInParams,
+			},
+		},
+	}
+	t.Cleanup(func() { require.NoError(t, a.h.state.Close()) })
+
+	a.authCache = expirable.NewLRU[string, AuthInfo](capacity+authCacheSSHReserve, func(_ string, info AuthInfo) {
+		if info.Registration {
+			a.authCacheRegistrations.Add(-1)
+		}
+	}, time.Minute)
+	t.Cleanup(a.authCache.Close)
+
+	return a
+}
+
+func oidcPendingAuthRequest(t *testing.T, authID types.AuthID) *http.Request {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/register/"+authID.String(), nil)
+	rctx := chi.NewRouteContext()
+	rctx.URLParams.Add("auth_id", authID.String())
+
+	return req.WithContext(context.WithValue(req.Context(), chi.RouteCtxKey, rctx))
+}
+
+func startOIDCPendingAuth(t *testing.T, a *AuthProviderOIDC, registration bool) (*httptest.ResponseRecorder, types.AuthID) {
+	t.Helper()
+
+	authID := types.MustAuthID()
+
+	authReq := types.NewSSHCheckAuthRequest(1, 2)
+	if registration {
+		authReq = types.NewRegisterAuthRequest(&types.RegistrationData{Hostname: "pending-oidc"})
+	}
+
+	require.NoError(t, a.h.state.SetAuthCacheEntry(authID, authReq))
+
+	rec := httptest.NewRecorder()
+	a.authHandler(rec, oidcPendingAuthRequest(t, authID), registration)
+
+	return rec, authID
+}
+
+func TestOIDCPendingAdmissionPreservesSessions(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	a.cfg.PKCE.Enabled = true
+	a.cfg.PKCE.Method = types.PKCEMethodS256
+
+	states := make(map[string]AuthInfo)
+
+	for _, registration := range []bool{true, false} {
+		rec, authID := startOIDCPendingAuth(t, a, registration)
+		require.Equal(t, http.StatusFound, rec.Code)
+		location, err := rec.Result().Location()
+		require.NoError(t, err)
+
+		state := location.Query().Get("state")
+		info, ok := a.authCache.Peek(state)
+		require.True(t, ok)
+		require.NotNil(t, info.Verifier)
+		assert.Equal(t, authID, info.AuthID)
+		assert.Equal(t, registration, info.Registration)
+		assert.Equal(t, "S256", location.Query().Get("code_challenge_method"))
+
+		states[state] = info
+	}
+
+	rejected, _ := startOIDCPendingAuth(t, a, true)
+	assert.Equal(t, http.StatusServiceUnavailable, rejected.Code,
+		"a full pending-state cache must reject admission instead of evicting an in-flight login")
+	assert.Equal(t, "10", rejected.Header().Get("Retry-After"))
+	assert.Empty(t, rejected.Header().Get("Location"))
+	assert.Empty(t, rejected.Header().Values("Set-Cookie"))
+
+	for state, want := range states {
+		assert.Equal(t, &want, a.getAuthInfoFromState(state), "an admitted session must survive a rejected admission")
+	}
+
+	assert.Zero(t, a.authCache.Len())
+}
+
+func TestOIDCPendingAdmissionReservesSSHCapacity(t *testing.T) {
+	a := newOIDCPendingProvider(t, 2)
+	registrations := make(map[string]types.AuthID)
+
+	for range 2 {
+		rec, authID := startOIDCPendingAuth(t, a, true)
+		require.Equal(t, http.StatusFound, rec.Code)
+		location, err := rec.Result().Location()
+		require.NoError(t, err)
+
+		registrations[location.Query().Get("state")] = authID
+	}
+
+	rejected, _ := startOIDCPendingAuth(t, a, true)
+	assert.Equal(t, http.StatusServiceUnavailable, rejected.Code)
+	assert.Empty(t, rejected.Header().Values("Set-Cookie"))
+
+	for range 64 {
+		rec, authID := startOIDCPendingAuth(t, a, false)
+		require.Equal(t, http.StatusFound, rec.Code, "registration load must leave the SSH reserve available")
+		location, err := rec.Result().Location()
+		require.NoError(t, err)
+
+		info, ok := a.authCache.Peek(location.Query().Get("state"))
+		require.True(t, ok)
+		assert.Equal(t, authID, info.AuthID)
+		assert.False(t, info.Registration)
+	}
+
+	full, _ := startOIDCPendingAuth(t, a, false)
+	assert.Equal(t, http.StatusServiceUnavailable, full.Code, "the SSH reserve must also be bounded")
+	assert.Empty(t, full.Header().Get("Location"))
+	assert.Empty(t, full.Header().Values("Set-Cookie"))
+	assert.Equal(t, 66, a.authCache.Len())
+
+	for state, authID := range registrations {
+		info := a.getAuthInfoFromState(state)
+		require.NotNil(t, info, "admission at the total bound must not evict an earlier registration")
+		assert.Equal(t, authID, info.AuthID)
+	}
+}
+
+func TestOIDCPendingAdmissionConcurrent(t *testing.T) {
+	a := newOIDCPendingProvider(t, 2)
+
+	type result struct {
+		authID types.AuthID
+		rec    *httptest.ResponseRecorder
+	}
+
+	results := make(chan result, 4)
+	start := make(chan struct{})
+
+	var workers sync.WaitGroup
+
+	for range 4 {
+		authID := types.MustAuthID()
+		require.NoError(t, a.h.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{Hostname: "concurrent-oidc"})))
+		req := oidcPendingAuthRequest(t, authID)
+
+		workers.Go(func() {
+			<-start
+
+			rec := httptest.NewRecorder()
+			a.RegisterHandler(rec, req)
+
+			results <- result{authID: authID, rec: rec}
+		})
+	}
+
+	close(start)
+	workers.Wait()
+	close(results)
+
+	accepted := make(map[string]types.AuthID)
+	rejected := 0
+
+	for result := range results {
+		if result.rec.Code == http.StatusFound {
+			location, err := result.rec.Result().Location()
+			require.NoError(t, err)
+
+			accepted[location.Query().Get("state")] = result.authID
+		} else {
+			assert.Equal(t, http.StatusServiceUnavailable, result.rec.Code)
+			assert.Empty(t, result.rec.Header().Get("Location"))
+			assert.Empty(t, result.rec.Header().Values("Set-Cookie"))
+
+			rejected++
+		}
+	}
+
+	require.Len(t, accepted, 2, "concurrent admission must respect the pending-state bound")
+	assert.Equal(t, 2, rejected)
+	assert.Equal(t, 2, a.authCache.Len())
+
+	for state, authID := range accepted {
+		info := a.getAuthInfoFromState(state)
+		require.NotNil(t, info, "every successful admission must remain usable")
+		assert.Equal(t, authID, info.AuthID)
+	}
+}
+
+func TestOIDCPendingAdmissionDuplicateState(t *testing.T) {
+	a := newOIDCPendingProvider(t, 2)
+	verifier := "original-verifier"
+	info := AuthInfo{AuthID: types.MustAuthID(), Verifier: &verifier, Registration: true}
+	require.True(t, a.addAuthInfoToState("same-state", info))
+	assert.False(t, a.addAuthInfoToState("same-state", AuthInfo{AuthID: types.MustAuthID()}),
+		"a duplicate state must not replace the auth ID, flow type, or PKCE verifier")
+	assert.Equal(t, &info, a.getAuthInfoFromState("same-state"))
+	assert.Zero(t, a.authCacheRegistrations.Load())
+}
+
+func TestOIDCPendingAdmissionSSHUsesNormalSlots(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	for i := range 65 {
+		require.True(t, a.addAuthInfoToState(fmt.Sprintf("ssh-state-%d", i), AuthInfo{AuthID: types.MustAuthID()}),
+			"SSH checks may use the regular slots as well as their reserve")
+	}
+
+	assert.False(t, a.addAuthInfoToState("one-too-many", AuthInfo{AuthID: types.MustAuthID()}))
+	assert.Equal(t, 65, a.authCache.Len())
+	assert.Zero(t, a.authCacheRegistrations.Load())
+}
+
+func TestOIDCPendingAdmissionRequiresAuthRequest(t *testing.T) {
+	for _, registration := range []bool{true, false} {
+		for _, existing := range []bool{false, true} {
+			t.Run(fmt.Sprintf("registration=%t/existing=%t", registration, existing), func(t *testing.T) {
+				a := newOIDCPendingProvider(t, 1)
+				authID := types.MustAuthID()
+				wantStatus := http.StatusGone
+
+				if existing {
+					// A registration cannot authorize an SSH check, or vice versa.
+					authReq := types.NewRegisterAuthRequest(&types.RegistrationData{Hostname: "wrong-flow"})
+					if registration {
+						authReq = types.NewSSHCheckAuthRequest(1, 2)
+					}
+
+					require.NoError(t, a.h.state.SetAuthCacheEntry(authID, authReq))
+
+					wantStatus = http.StatusBadRequest
+				}
+
+				rec := httptest.NewRecorder()
+				a.authHandler(rec, oidcPendingAuthRequest(t, authID), registration)
+				assert.Equal(t, wantStatus, rec.Code)
+				assert.Empty(t, rec.Header().Get("Location"))
+				assert.Empty(t, rec.Header().Values("Set-Cookie"))
+				assert.Zero(t, a.authCache.Len(), "invalid auth requests must not reserve OIDC capacity")
+			})
+		}
+	}
+}
+
+func TestOIDCPendingAdmissionInvalidPKCE(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	a.cfg.PKCE.Enabled = true
+	a.cfg.PKCE.Method = "invalid"
+
+	rec, _ := startOIDCPendingAuth(t, a, true)
+	assert.Equal(t, http.StatusInternalServerError, rec.Code)
+	assert.Empty(t, rec.Header().Get("Location"))
+	assert.Empty(t, rec.Header().Values("Set-Cookie"))
+	assert.Zero(t, a.authCache.Len())
+
+	a.cfg.PKCE.Method = types.PKCEMethodPlain
+	rec, _ = startOIDCPendingAuth(t, a, true)
+	assert.Equal(t, http.StatusFound, rec.Code, "a preparation error must leave capacity available")
+}
+
+func TestOIDCPendingAdmissionReclaimsExpired(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		a := newOIDCPendingProvider(t, 2)
+		a.authCache.Close()
+
+		a.authCache = expirable.NewLRU[string, AuthInfo](66, func(_ string, info AuthInfo) {
+			if info.Registration {
+				a.authCacheRegistrations.Add(-1)
+			}
+		}, time.Second)
+		defer a.authCache.Close()
+
+		old, _ := startOIDCPendingAuth(t, a, true)
+		require.Equal(t, http.StatusFound, old.Code)
+		oldURL, err := old.Result().Location()
+		require.NoError(t, err)
+
+		<-time.After(time.Millisecond)
+
+		live, liveID := startOIDCPendingAuth(t, a, true)
+		require.Equal(t, http.StatusFound, live.Code)
+		liveURL, err := live.Result().Location()
+		require.NoError(t, err)
+
+		// Both entries share a cleanup bucket, which waits for the newer expiry.
+		<-time.After(time.Second - time.Millisecond + time.Nanosecond)
+		require.Equal(t, 2, a.authCache.Len(), "exercise expired entries that the sweeper has not removed")
+		_, ok := a.authCache.Peek(oldURL.Query().Get("state"))
+		require.False(t, ok)
+
+		next, _ := startOIDCPendingAuth(t, a, true)
+		assert.Equal(t, http.StatusFound, next.Code, "expired entries must not cause a false capacity rejection")
+		assert.Nil(t, a.getAuthInfoFromState(oldURL.Query().Get("state")))
+		info := a.getAuthInfoFromState(liveURL.Query().Get("state"))
+		require.NotNil(t, info, "reclaiming expiry must preserve the live session")
+		assert.Equal(t, liveID, info.AuthID)
+	})
+}
+
+func TestOIDCPendingAdmissionReclaimsSweptCapacity(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	a.authCache.Close()
+
+	synctest.Test(t, func(t *testing.T) {
+		a.authCache = expirable.NewLRU[string, AuthInfo](65, func(_ string, info AuthInfo) {
+			if info.Registration {
+				a.authCacheRegistrations.Add(-1)
+			}
+		}, time.Second)
+		defer a.authCache.Close()
+
+		require.True(t, a.addAuthInfoToState("expires", AuthInfo{Registration: true}))
+
+		<-time.After(time.Second + 10*time.Millisecond)
+		synctest.Wait()
+		assert.Zero(t, a.authCache.Len())
+		assert.Zero(t, a.authCacheRegistrations.Load(), "background expiry must return the registration budget")
+		assert.True(t, a.addAuthInfoToState("replacement", AuthInfo{Registration: true}))
+	})
+}
+
+func TestOIDCPendingCallbackReleasesOnError(t *testing.T) {
+	for _, pkce := range []bool{false, true} {
+		for _, missingToken := range []bool{false, true} {
+			t.Run(fmt.Sprintf("pkce=%t/missingToken=%t", pkce, missingToken), func(t *testing.T) {
+				a := newOIDCPendingProvider(t, 1)
+				a.cfg.PKCE.Enabled = pkce
+				a.cfg.PKCE.Method = types.PKCEMethodS256
+
+				var exchanges atomic.Int32
+
+				tokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+					exchanges.Add(1)
+					w.Header().Set("Content-Type", "application/json")
+
+					if missingToken {
+						_, _ = io.WriteString(w, `{"access_token":"test-token","token_type":"Bearer"}`)
+					} else {
+						w.WriteHeader(http.StatusBadRequest)
+						_, _ = io.WriteString(w, `{"error":"invalid_grant"}`)
+					}
+				}))
+				t.Cleanup(tokenServer.Close)
+				a.oauth2Config.Endpoint.TokenURL = tokenServer.URL
+
+				started, authID := startOIDCPendingAuth(t, a, true)
+				require.Equal(t, http.StatusFound, started.Code)
+				location, err := started.Result().Location()
+				require.NoError(t, err)
+
+				state := location.Query().Get("state")
+				callbackURL := "/oidc/callback?code=test-code&state=" + state
+				callback := httptest.NewRequestWithContext(t.Context(), http.MethodGet, callbackURL, nil)
+				//nolint:gosec // G124: request cookies carry only name and value on the wire.
+				callback.AddCookie(&http.Cookie{Name: getCookieName("state", state), Value: state})
+
+				rec := httptest.NewRecorder()
+				a.OIDCCallbackHandler(rec, callback)
+
+				wantStatus := http.StatusForbidden
+				if missingToken {
+					wantStatus = http.StatusBadRequest
+				}
+
+				assert.Equal(t, wantStatus, rec.Code)
+				assert.Zero(t, a.authCache.Len(), "a callback error must release its pending-state slot")
+				_, pending := a.h.state.GetAuthCacheEntry(authID)
+				assert.True(t, pending, "consuming OIDC state must preserve the underlying auth request")
+
+				replay := httptest.NewRecorder()
+				a.OIDCCallbackHandler(replay, callback)
+				assert.Equal(t, http.StatusGone, replay.Code, "a failed callback must still consume its state")
+				assert.EqualValues(t, 1, exchanges.Load(), "a consumed state must not reach token exchange again")
+
+				next := httptest.NewRecorder()
+				a.RegisterHandler(next, oidcPendingAuthRequest(t, authID))
+				assert.Equal(t, http.StatusFound, next.Code, "the original auth request must support a fresh browser flow")
+				nextURL, err := next.Result().Location()
+				require.NoError(t, err)
+				assert.NotEqual(t, state, nextURL.Query().Get("state"))
+			})
+		}
+	}
+}
+
+func TestOIDCPendingCallbackProviderError(t *testing.T) {
+	for _, tc := range []struct {
+		name          string
+		providerError string
+		code          string
+	}{
+		{name: "cancelled", providerError: "access_denied"},
+		{name: "error with code", providerError: "access_denied", code: "unused-code"},
+		{name: "unknown error", providerError: strings.Repeat("untrusted-provider-error-", 2048)},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			a := newOIDCPendingProvider(t, 1)
+			a.cfg.PKCE.Enabled = true
+			a.cfg.PKCE.Method = types.PKCEMethodS256
+
+			var exchanges atomic.Int32
+
+			tokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				exchanges.Add(1)
+				w.Header().Set("Content-Type", "application/json")
+				w.WriteHeader(http.StatusBadRequest)
+				_, _ = io.WriteString(w, `{"error":"invalid_grant"}`)
+			}))
+			t.Cleanup(tokenServer.Close)
+			a.oauth2Config.Endpoint.TokenURL = tokenServer.URL
+
+			started, authID := startOIDCPendingAuth(t, a, true)
+			require.Equal(t, http.StatusFound, started.Code)
+			location, err := started.Result().Location()
+			require.NoError(t, err)
+
+			state := location.Query().Get("state")
+
+			full := httptest.NewRecorder()
+			a.RegisterHandler(full, oidcPendingAuthRequest(t, authID))
+			require.Equal(t, http.StatusServiceUnavailable, full.Code)
+
+			query := url.Values{
+				"error":             {tc.providerError},
+				"error_description": {strings.Repeat("untrusted-provider-description-", 2048)},
+				"state":             {state},
+				"code":              {tc.code},
+			}
+
+			ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
+			defer cancel()
+
+			callback := httptest.NewRequestWithContext(ctx, http.MethodGet, "/oidc/callback?"+query.Encode(), nil)
+			//nolint:gosec // G124: request cookies carry only name and value on the wire.
+			callback.AddCookie(&http.Cookie{Name: getCookieName("state", state), Value: state})
+
+			rec := httptest.NewRecorder()
+			a.OIDCCallbackHandler(rec, callback)
+			assert.Equal(t, http.StatusForbidden, rec.Code)
+			assert.NotContains(t, rec.Body.String(), "untrusted-provider-description")
+			assert.NotContains(t, rec.Body.String(), "untrusted-provider-error")
+			assert.Less(t, rec.Body.Len(), 16*1024, "provider input must not inflate the error response")
+			assert.Zero(t, a.authCache.Len(), "provider cancellation must release its pending-state slot")
+			assert.Zero(t, a.authCacheRegistrations.Load())
+			_, pending := a.h.state.GetAuthCacheEntry(authID)
+			assert.True(t, pending, "provider cancellation must preserve the underlying auth request")
+
+			replay := httptest.NewRecorder()
+			a.OIDCCallbackHandler(replay, callback)
+			assert.Equal(t, http.StatusGone, replay.Code, "a provider-error callback must consume its state exactly once")
+			assert.Zero(t, exchanges.Load(), "provider-error callbacks must never exchange a code")
+
+			next := httptest.NewRecorder()
+			a.RegisterHandler(next, oidcPendingAuthRequest(t, authID))
+			require.Equal(t, http.StatusFound, next.Code, "cancellation must free capacity for a fresh login")
+			nextURL, err := next.Result().Location()
+			require.NoError(t, err)
+			assert.NotEqual(t, state, nextURL.Query().Get("state"))
+		})
+	}
+}
+
+func TestOIDCPendingCallbackProviderErrorValidation(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	started, authID := startOIDCPendingAuth(t, a, true)
+	require.Equal(t, http.StatusFound, started.Code)
+	location, err := started.Result().Location()
+	require.NoError(t, err)
+
+	state := location.Query().Get("state")
+	cookieName := getCookieName("state", state)
+	cookie := cookieName + "=" + state
+	providerError := "error=access_denied&state=" + state
+
+	for _, tc := range []struct {
+		name   string
+		query  string
+		cookie string
+		status int
+	}{
+		{name: "missing state", query: "error=access_denied", cookie: cookie, status: http.StatusBadRequest},
+		{name: "short state", query: "error=access_denied&state=abc", cookie: "state_abc=abc", status: http.StatusBadRequest},
+		{name: "missing code and error", query: "state=" + state, cookie: cookie, status: http.StatusBadRequest},
+		{name: "empty error", query: "error=&state=" + state, cookie: cookie, status: http.StatusBadRequest},
+		{name: "description only", query: "error_description=cancelled&state=" + state, cookie: cookie, status: http.StatusBadRequest},
+		{name: "missing cookie", query: providerError, status: http.StatusBadRequest},
+		{name: "invalid cookie", query: providerError, cookie: cookieName + "=\"unterminated", status: http.StatusBadRequest},
+		{name: "empty cookie", query: providerError, cookie: cookieName + "=", status: http.StatusForbidden},
+		{name: "wrong cookie value", query: providerError, cookie: cookieName + "=different-state", status: http.StatusForbidden},
+		{name: "wrong cookie name", query: providerError, cookie: "state_other=" + state, status: http.StatusBadRequest},
+		{name: "unknown state", query: "error=access_denied&state=unknown-state", cookie: "state_unknow=unknown-state", status: http.StatusGone},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			callback := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/oidc/callback?"+tc.query, nil)
+			callback.Header.Set("Cookie", tc.cookie)
+
+			rec := httptest.NewRecorder()
+			a.OIDCCallbackHandler(rec, callback)
+			assert.Equal(t, tc.status, rec.Code)
+
+			info, ok := a.authCache.Peek(state)
+			require.True(t, ok, "invalid callbacks must preserve the pending state")
+			assert.Equal(t, authID, info.AuthID)
+
+			full := httptest.NewRecorder()
+			a.RegisterHandler(full, oidcPendingAuthRequest(t, authID))
+			assert.Equal(t, http.StatusServiceUnavailable, full.Code, "invalid callbacks must not free another browser's slot")
+		})
+	}
+}
+
+func TestOIDCPendingCallbackRequiresState(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+
+	var exchanges atomic.Int32
+
+	tokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		exchanges.Add(1)
+		w.WriteHeader(http.StatusBadRequest)
+	}))
+	t.Cleanup(tokenServer.Close)
+	a.oauth2Config.Endpoint.TokenURL = tokenServer.URL
+
+	const state = "unknown-oidc-state"
+
+	callback := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/oidc/callback?code=test-code&state="+state, nil)
+	//nolint:gosec // G124: request cookies carry only name and value on the wire.
+	callback.AddCookie(&http.Cookie{Name: getCookieName("state", state), Value: state})
+
+	rec := httptest.NewRecorder()
+	a.OIDCCallbackHandler(rec, callback)
+	assert.Equal(t, http.StatusGone, rec.Code)
+	assert.Zero(t, exchanges.Load(), "unknown state must be rejected before token exchange even without PKCE")
+}
+
+func TestOIDCPendingCallbackConcurrent(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	a.cfg.PKCE.Enabled = true
+	a.cfg.PKCE.Method = types.PKCEMethodS256
+	entered := make(chan struct{}, 2)
+	release := make(chan struct{})
+
+	var releaseOnce sync.Once
+
+	unblock := func() { releaseOnce.Do(func() { close(release) }) }
+
+	var exchanges atomic.Int32
+
+	tokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		exchanges.Add(1)
+
+		entered <- struct{}{}
+
+		<-release
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = io.WriteString(w, `{"error":"invalid_grant"}`)
+	}))
+	t.Cleanup(tokenServer.Close)
+	t.Cleanup(unblock)
+
+	a.oauth2Config.Endpoint.TokenURL = tokenServer.URL
+
+	started, _ := startOIDCPendingAuth(t, a, true)
+	require.Equal(t, http.StatusFound, started.Code)
+	location, err := started.Result().Location()
+	require.NoError(t, err)
+
+	state := location.Query().Get("state")
+	callbackURL := "/oidc/callback?code=test-code&state=" + state
+	first := httptest.NewRequestWithContext(t.Context(), http.MethodGet, callbackURL, nil)
+	//nolint:gosec // G124: request cookies carry only name and value on the wire.
+	first.AddCookie(&http.Cookie{Name: getCookieName("state", state), Value: state})
+	second := first.Clone(t.Context())
+	firstRec, secondRec := httptest.NewRecorder(), httptest.NewRecorder()
+	firstDone, secondDone := make(chan struct{}), make(chan struct{})
+
+	go func() {
+		a.OIDCCallbackHandler(firstRec, first)
+		close(firstDone)
+	}()
+
+	select {
+	case <-entered:
+	case <-firstDone:
+		t.Fatal("the first callback must reach token exchange")
+	case <-time.After(5 * time.Second):
+		t.Fatal("the first callback did not reach token exchange")
+	}
+
+	go func() {
+		a.OIDCCallbackHandler(secondRec, second)
+		close(secondDone)
+	}()
+
+	select {
+	case <-secondDone:
+	case <-entered:
+		// Release both requests before asserting the single-exchange invariant.
+	case <-time.After(5 * time.Second):
+		t.Fatal("the duplicate callback did not finish or reach token exchange")
+	}
+
+	unblock()
+	<-firstDone
+	<-secondDone
+	assert.Equal(t, http.StatusForbidden, firstRec.Code)
+	assert.Equal(t, http.StatusGone, secondRec.Code)
+	assert.EqualValues(t, 1, exchanges.Load(), "only one callback may claim a pending state")
+	assert.Zero(t, a.authCache.Len())
+}
+
+func TestOIDCPendingCallbackCookieBinding(t *testing.T) {
+	a := newOIDCPendingProvider(t, 1)
+	started, _ := startOIDCPendingAuth(t, a, true)
+	require.Equal(t, http.StatusFound, started.Code)
+	location, err := started.Result().Location()
+	require.NoError(t, err)
+
+	state := location.Query().Get("state")
+
+	for _, value := range []string{"", "different-state"} {
+		callback := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/oidc/callback?code=test-code&state="+state, nil)
+		wantStatus := http.StatusBadRequest
+
+		if value != "" {
+			//nolint:gosec // G124: request cookies carry only name and value on the wire.
+			callback.AddCookie(&http.Cookie{Name: getCookieName("state", state), Value: value})
+
+			wantStatus = http.StatusForbidden
+		}
+
+		rec := httptest.NewRecorder()
+		a.OIDCCallbackHandler(rec, callback)
+		assert.Equal(t, wantStatus, rec.Code)
+
+		_, ok := a.authCache.Peek(state)
+		assert.True(t, ok, "an unbound callback must not consume another browser's pending state")
+	}
+}
+
+func TestOIDCPendingCallbackPKCEFlow(t *testing.T) {
+	for _, method := range []string{"", types.PKCEMethodPlain, types.PKCEMethodS256} {
+		t.Run("method="+method, func(t *testing.T) {
+			b := newOIDCBrowser(t)
+			a, ok := b.app.authProvider.(*AuthProviderOIDC)
+			require.True(t, ok)
+
+			a.cfg.PKCE.Enabled = method != ""
+			a.cfg.PKCE.Method = method
+			a.authCache.Resize(65)
+
+			authID, registerURL := b.pendingNode(t)
+
+			status, _, body := b.get(t, registerURL)
+			require.Equal(t, http.StatusOK, status)
+			assert.Contains(t, body, "Confirm node registration")
+			assert.Zero(t, a.authCache.Len(), "successful callbacks must release pending OIDC capacity")
+
+			authReq, ok := b.app.state.GetAuthCacheEntry(authID)
+			require.True(t, ok, "consuming OIDC state must preserve the registration for confirmation")
+			assert.NotNil(t, authReq.PendingConfirmation())
+		})
+	}
+}
+
+func TestOIDCPendingCallbackPreservesSSHVerdict(t *testing.T) {
+	b := newOIDCBrowser(t)
+	a, ok := b.app.authProvider.(*AuthProviderOIDC)
+	require.True(t, ok)
+
+	principal := mockoidc.DefaultUser()
+	user, _, err := a.createOrUpdateUserFromClaim(&types.OIDCClaims{
+		Iss:           b.idp.Issuer(),
+		Sub:           principal.Subject,
+		Username:      principal.PreferredUsername,
+		Email:         principal.Email,
+		EmailVerified: true,
+	})
+	require.NoError(t, err)
+
+	src := b.app.state.CreateNodeForTest(user, "oidc-ssh-source")
+	b.app.state.PutNodeInStoreForTest(*src)
+
+	authID := types.MustAuthID()
+	authReq := types.NewSSHCheckAuthRequest(src.ID, 2)
+	require.NoError(t, b.app.state.SetAuthCacheEntry(authID, authReq))
+
+	status, _, body := b.get(t, a.AuthURL(authID))
+	require.Equal(t, http.StatusOK, status)
+	assert.Contains(t, body, "You may return to your terminal.")
+	assert.Zero(t, a.authCache.Len())
+
+	cached, ok := b.app.state.GetAuthCacheEntry(authID)
+	require.True(t, ok, "completing OIDC must preserve the SSH verdict for its polling client")
+	assert.Same(t, authReq, cached)
+
+	select {
+	case verdict := <-cached.WaitForAuth():
+		assert.True(t, verdict.Accept())
+	default:
+		t.Fatal("successful SSH authentication must make its verdict available")
+	}
+}
+
+func TestOIDCPendingCallbackNonceBinding(t *testing.T) {
+	b := newOIDCBrowser(t)
+	a, ok := b.app.authProvider.(*AuthProviderOIDC)
+	require.True(t, ok)
+
+	b.client.CheckRedirect = func(req *http.Request, _ []*http.Request) error {
+		if req.URL.Path == "/oidc/callback" {
+			return http.ErrUseLastResponse
+		}
+
+		return nil
+	}
+	_, registerURL := b.pendingNode(t)
+	//nolint:noctx,bodyclose // local test browser; closed below
+	response, err := b.client.Get(registerURL)
+	require.NoError(t, err)
+
+	defer response.Body.Close()
+
+	callbackURL, err := response.Location()
+	require.NoError(t, err)
+	require.Equal(t, "/oidc/callback", callbackURL.Path)
+
+	callback := httptest.NewRequestWithContext(t.Context(), http.MethodGet, callbackURL.String(), nil)
+	for _, cookie := range b.client.Jar.Cookies(callbackURL) {
+		if strings.HasPrefix(cookie.Name, "nonce_") {
+			cookie.Value = "wrong-nonce"
+		}
+
+		callback.AddCookie(cookie)
+	}
+
+	rec := httptest.NewRecorder()
+	a.OIDCCallbackHandler(rec, callback)
+	assert.Equal(t, http.StatusForbidden, rec.Code)
+	assert.Zero(t, a.authCache.Len(), "nonce rejection must release the consumed pending state")
+}
+
 // TestClearOIDCCallbackCookie asserts the cookie is expired (negative MaxAge) on
 // the same path it was set with, so the browser drops it.
 func TestClearOIDCCallbackCookie(t *testing.T) {
@@ -265,14 +1020,14 @@ func TestSetCSRFCookieSecure(t *testing.T) {
 
 	secureRec := httptest.NewRecorder()
 	secureProvider := &AuthProviderOIDC{serverURL: "https://hs.example.com"}
-	secureProvider.setCSRFCookie(secureRec, r, "state")
+	secureProvider.setCSRFCookie(secureRec, r, "state", "state-value")
 	require.Len(t, secureRec.Result().Cookies(), 1)
 	assert.True(t, secureRec.Result().Cookies()[0].Secure,
 		"https server_url must set Secure even when req.TLS is nil (proxy case)")
 
 	plainRec := httptest.NewRecorder()
 	plainProvider := &AuthProviderOIDC{serverURL: "http://hs.example.com"}
-	plainProvider.setCSRFCookie(plainRec, r, "state")
+	plainProvider.setCSRFCookie(plainRec, r, "state", "state-value")
 	require.Len(t, plainRec.Result().Cookies(), 1)
 	assert.False(t, plainRec.Result().Cookies()[0].Secure,
 		"plain-http server_url without req.TLS must not set Secure")
@@ -379,11 +1134,11 @@ func (b *oidcBrowser) pendingNode(t *testing.T) (types.AuthID, string) {
 	t.Helper()
 
 	authID := types.MustAuthID()
-	b.app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, b.app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: key.NewMachine().Public(),
 		NodeKey:    key.NewNode().Public(),
 		Hostname:   "reload-victim",
-	}))
+	})))
 
 	return authID, b.publicURL + "/register/" + authID.String()
 }
@@ -710,10 +1465,10 @@ func TestOIDCRegistrationRejectsOversizedMergedHostinfo(t *testing.T) {
 
 	hostname := strings.Repeat("h", types.MaxHostnameLength)
 	authID := types.MustAuthID()
-	app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, app.state.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: node.MachineKey, NodeKey: key.NewNode().Public(),
 		Hostname: hostname, Hostinfo: &tailcfg.Hostinfo{Hostname: hostname},
-	}))
+	})))
 
 	provider := &AuthProviderOIDC{h: app}
 	changed, err := provider.handleRegistration(user, authID, nil)

hscontrol/state/auth_cache_test.go

@@ -5,6 +5,7 @@ import (
 	"runtime"
 	"runtime/pprof"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -45,10 +46,8 @@ func requireExpiredVerdict(t *testing.T, entry *types.AuthRequest) {
 	}
 }
 
-// TestAuthCacheBoundedLRU verifies that the registration auth cache is
-// bounded by a maximum entry count, that exceeding the maxEntries evicts the
-// oldest entry, and that the eviction callback resolves the parked
-// AuthRequest with ErrRegistrationExpired so any waiting goroutine wakes.
+// TestAuthCacheBoundedLRU checks the underlying library's eviction callback.
+// State admission prevents this eviction behavior for pending auth requests.
 func TestAuthCacheBoundedLRU(t *testing.T) {
 	const maxEntries = 4
 
@@ -97,6 +96,171 @@ func TestAuthCacheTTLExpiry(t *testing.T) {
 	assert.False(t, ok, "expired entry must be gone from the cache")
 }
 
+func TestAuthCacheAdmissionPreservesPending(t *testing.T) {
+	cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+	cfg.Tuning.RegisterCacheMaxEntries = 1
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+	id := types.MustAuthID()
+	pending := types.NewAuthRequest()
+	require.NoError(t, s.SetAuthCacheEntry(id, pending))
+
+	otherID := types.MustAuthID()
+	require.ErrorIs(t, s.SetAuthCacheEntry(otherID, types.NewAuthRequest()), ErrAuthCacheFull)
+
+	got, ok := s.GetAuthCacheEntry(id)
+	require.True(t, ok, "new work must not displace an admitted authentication")
+	require.Same(t, pending, got)
+
+	_, ok = s.GetAuthCacheEntry(otherID)
+	require.False(t, ok, "new work must be refused at capacity")
+
+	select {
+	case <-pending.WaitForAuth():
+		t.Fatal("capacity refusal must not finish an existing authentication")
+	default:
+	}
+}
+
+func TestAuthCacheAdmissionPreservesIdentity(t *testing.T) {
+	cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+	id := types.MustAuthID()
+	pending := types.NewAuthRequest()
+	require.NoError(t, s.SetAuthCacheEntry(id, pending))
+	require.NoError(t, s.SetAuthCacheEntry(id, pending))
+	require.ErrorIs(t, s.SetAuthCacheEntry(id, types.NewAuthRequest()), ErrAuthCacheEntryExists)
+
+	got, ok := s.GetAuthCacheEntry(id)
+	require.True(t, ok)
+	require.Same(t, pending, got, "an existing auth ID must keep its original request and waiter")
+}
+
+func TestAuthCacheAdmissionConcurrent(t *testing.T) {
+	cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+	cfg.Tuning.RegisterCacheMaxEntries = 2
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+	id := types.MustAuthID()
+	pending := types.NewAuthRequest()
+	require.NoError(t, s.SetAuthCacheEntry(id, pending))
+
+	var wg sync.WaitGroup
+
+	results := make(chan error, 4)
+
+	for range 4 {
+		wg.Go(func() {
+			results <- s.SetAuthCacheEntry(types.MustAuthID(), types.NewAuthRequest())
+		})
+	}
+
+	wg.Wait()
+	close(results)
+
+	var accepted int
+
+	for err := range results {
+		if err == nil {
+			accepted++
+		} else {
+			require.ErrorIs(t, err, ErrAuthCacheFull)
+		}
+	}
+
+	require.Equal(t, 1, accepted)
+
+	got, ok := s.GetAuthCacheEntry(id)
+	require.True(t, ok, "concurrent admission must preserve existing transactions")
+	require.Same(t, pending, got)
+	require.Equal(t, 2, s.authCache.Len())
+}
+
+func TestAuthCacheAdmissionReservesSSHCapacity(t *testing.T) {
+	cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+	cfg.Tuning.RegisterCacheMaxEntries = 1
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+	registrationID := types.MustAuthID()
+	require.NoError(t, s.SetAuthCacheEntry(registrationID, types.NewAuthRequest()))
+
+	sshID := types.MustAuthID()
+	require.NoError(t, s.SetAuthCacheEntry(sshID, types.NewSSHCheckAuthRequest(1, 2)),
+		"registration capacity must not consume the SSH reservation")
+	require.ErrorIs(t, s.SetAuthCacheEntry(types.MustAuthID(), types.NewAuthRequest()), ErrAuthCacheFull)
+	_, ok := s.GetAuthCacheEntry(registrationID)
+	require.True(t, ok)
+	_, ok = s.GetAuthCacheEntry(sshID)
+	require.True(t, ok)
+}
+
+func TestAuthCacheAdmissionBoundsRegistrationPayload(t *testing.T) {
+	cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+	id := types.MustAuthID()
+	request := types.NewRegisterAuthRequest(&types.RegistrationData{
+		Hostname: strings.Repeat("x", types.MaxRegistrationPayloadBytes),
+	})
+	require.ErrorIs(t, s.SetAuthCacheEntry(id, request), types.ErrRegistrationPayloadTooLarge)
+	_, ok := s.GetAuthCacheEntry(id)
+	require.False(t, ok)
+	require.Zero(t, s.authCache.Len())
+	require.Zero(t, s.authCacheRegularEntries.Load())
+}
+
+func TestAuthCacheAdmissionDoesNotReuseExpiredRequest(t *testing.T) {
+	cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+	cfg.Tuning.RegisterCacheExpiration = 50 * time.Millisecond
+	s, err := NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+	id := types.MustAuthID()
+	request := types.NewAuthRequest()
+	require.NoError(t, s.SetAuthCacheEntry(id, request))
+	requireExpiredVerdict(t, request)
+	require.Error(t, s.SetAuthCacheEntry(id, request), "expired waiters cannot be reused as new pending requests")
+}
+
+func TestAuthCacheAdmissionReleasesCapacity(t *testing.T) {
+	for _, removal := range []string{"consumption", "expiry"} {
+		t.Run(removal, func(t *testing.T) {
+			cfg := persistTestConfig(t.TempDir() + "/headscale.db")
+			cfg.Tuning.RegisterCacheMaxEntries = 1
+			cfg.Tuning.RegisterCacheExpiration = 50 * time.Millisecond
+			s, err := NewState(cfg)
+			require.NoError(t, err)
+			t.Cleanup(func() { require.NoError(t, s.Close()) })
+
+			id := types.MustAuthID()
+			pending := types.NewAuthRequest()
+			require.NoError(t, s.SetAuthCacheEntry(id, pending))
+
+			if removal == "consumption" {
+				pending.FinishAuth(types.AuthVerdict{})
+				s.DeleteAuthCacheEntry(id)
+			} else {
+				requireExpiredVerdict(t, pending)
+			}
+
+			require.NoError(t, s.SetAuthCacheEntry(types.MustAuthID(), types.NewAuthRequest()))
+			require.Equal(t, int64(1), s.authCacheRegularEntries.Load())
+		})
+	}
+}
+
 func goroutineID() string {
 	buf := make([]byte, 64)
 	n := runtime.Stack(buf, false)
@@ -138,11 +302,12 @@ func TestStateCloseStopsAuthCacheSweeper(t *testing.T) {
 	require.Equal(t, 1, authCacheSweepers(t, gid), "NewState must start a sweeper")
 
 	pending := types.NewAuthRequest()
-	s.SetAuthCacheEntry(types.MustAuthID(), pending)
+	require.NoError(t, s.SetAuthCacheEntry(types.MustAuthID(), pending))
 
 	require.NoError(t, s.Close())
 
 	requireExpiredVerdict(t, pending)
+	require.ErrorIs(t, s.SetAuthCacheEntry(types.MustAuthID(), types.NewAuthRequest()), ErrAuthCacheClosed)
 
 	assert.Eventually(t, func() bool {
 		return authCacheSweepers(t, gid) == 0

hscontrol/state/auth_tagged_expiry_test.go

@@ -81,7 +81,7 @@ func TestTaggedReauthKeepsNilExpiry(t *testing.T) {
 	}
 
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData))
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData)))
 
 	finalNode, _, err := s.HandleNodeFromAuthPath(
 		authID,
@@ -400,12 +400,12 @@ func TestAuthPathOnTaggedMachine(t *testing.T) {
 		t.Helper()
 
 		authID := types.MustAuthID()
-		s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+		require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 			MachineKey: mk,
 			NodeKey:    nodeKey,
 			Hostname:   "multi",
 			Hostinfo:   &tailcfg.Hostinfo{Hostname: "multi"},
-		}))
+		})))
 
 		node, _, err := s.HandleNodeFromAuthPath(authID, types.UserID(user.ID), nil, util.RegisterMethodOIDC)
 
@@ -443,12 +443,12 @@ func TestUserKeyOnLoginJoinedTaggedNode(t *testing.T) {
 	nodeKey := key.NewNode().Public()
 
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: machineKey,
 		NodeKey:    nodeKey,
 		Hostname:   "login-node",
 		Hostinfo:   &tailcfg.Hostinfo{Hostname: "login-node"},
-	}))
+	})))
 	joined, _, err := s.HandleNodeFromAuthPath(authID, types.UserID(owner.ID), nil, util.RegisterMethodOIDC)
 	require.NoError(t, err)
 
@@ -599,7 +599,7 @@ func (n seededTaggedNode) reauth(t *testing.T, authUser *types.User, requestTags
 	rd.Expiry = clientExpiry
 
 	authID := types.MustAuthID()
-	n.s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&rd))
+	require.NoError(t, n.s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&rd)))
 
 	node, _, err := n.s.HandleNodeFromAuthPath(
 		authID,
@@ -1001,7 +1001,7 @@ func TestIssue3371_TaggedNodeInteractiveReloginAfterLogout(t *testing.T) {
 		},
 	}
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData))
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(regData)))
 
 	relogged, _, err := s.HandleNodeFromAuthPath(
 		authID, types.UserID(user.ID), nil, util.RegisterMethodOIDC,

hscontrol/state/debug.go

@@ -181,7 +181,8 @@ func (s *State) DebugRegistrationCache() map[string]any {
 	return map[string]any{
 		"type":        "expirable-lru",
 		"expiration":  registerCacheExpiration.String(),
-		"max_entries": defaultRegisterCacheMaxEntries,
+		"max_entries": s.authCacheMaxEntries,
+		"ssh_reserve": authCacheSSHReserve,
 		"current_len": s.authCache.Len(),
 		"status":      "active",
 	}

hscontrol/state/persist_test.go

@@ -1387,11 +1387,11 @@ func TestAuthPathRejectsUnencodableExpiry(t *testing.T) {
 	user := s.CreateUserForTest("auth-path-user")
 	nodeKey := key.NewNode().Public()
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: key.NewMachine().Public(),
 		NodeKey:    nodeKey,
 		Hostname:   "auth-path-node",
-	}))
+	})))
 
 	// How go-oidc reads an exp claim in the year 10000.
 	beyond := time.Unix(253402300800, 0)
@@ -1419,11 +1419,11 @@ func TestAcceptedExpirySurvivesRestart(t *testing.T) {
 
 	user := s.CreateUserForTest("oidc-user")
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: key.NewMachine().Public(),
 		NodeKey:    key.NewNode().Public(),
 		Hostname:   "oidc-node",
-	}))
+	})))
 
 	oidcNode, _, err := s.HandleNodeFromAuthPath(
 		authID, types.UserID(user.ID), &east, util.RegisterMethodOIDC,
@@ -1648,12 +1648,12 @@ func TestReauthKeepsOperationalMetadata(t *testing.T) {
 		"precondition: the node serves its route")
 
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: node.MachineKey(),
 		NodeKey:    key.NewNode().Public(),
 		Hostname:   node.Hostname(),
 		Hostinfo:   &tailcfg.Hostinfo{Hostname: node.Hostname()},
-	}))
+	})))
 
 	relogged, _, err := s.HandleNodeFromAuthPath(
 		authID, node.TypedUserID(), nil, util.RegisterMethodOIDC,
@@ -1696,10 +1696,10 @@ func TestReauthRejectsOversizedMergedHostinfo(t *testing.T) {
 	require.NoError(t, types.ValidateHostinfo(&tailcfg.Hostinfo{Hostname: hostname}))
 
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: node.MachineKey(), NodeKey: key.NewNode().Public(),
 		Hostname: hostname, Hostinfo: &tailcfg.Hostinfo{Hostname: hostname},
-	}))
+	})))
 	_, _, err = s.HandleNodeFromAuthPath(authID, node.TypedUserID(), nil, util.RegisterMethodOIDC)
 	require.ErrorIs(t, err, types.ErrHostinfoTooLarge)
 
@@ -1734,11 +1734,11 @@ func TestReauthKeepsTaggedFutureExpiry(t *testing.T) {
 	require.True(t, node.IsTagged())
 
 	authID := types.MustAuthID()
-	s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+	require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 		MachineKey: node.MachineKey(), NodeKey: key.NewNode().Public(),
 		Hostname: node.Hostname(),
 		Hostinfo: &tailcfg.Hostinfo{Hostname: node.Hostname(), RequestTags: []string{"tag:ci"}},
-	}))
+	})))
 	relogged, _, err := s.HandleNodeFromAuthPath(authID, userID, nil, util.RegisterMethodOIDC)
 	require.NoError(t, err)
 	require.Equal(t, nodeID, relogged.ID())
@@ -1767,11 +1767,11 @@ func TestReauthReplacesReportedTags(t *testing.T) {
 	for _, tags := range [][]string{{"tag:a"}, nil, {"tag:b"}} {
 		want := slices.Clone(tags)
 		authID := types.MustAuthID()
-		s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
+		require.NoError(t, s.SetAuthCacheEntry(authID, types.NewRegisterAuthRequest(&types.RegistrationData{
 			MachineKey: node.MachineKey(), NodeKey: key.NewNode().Public(),
 			Hostname: node.Hostname(),
 			Hostinfo: &tailcfg.Hostinfo{Hostname: node.Hostname(), RequestTags: tags},
-		}))
+		})))
 		relogged, _, err := s.HandleNodeFromAuthPath(authID, userID, nil, util.RegisterMethodOIDC)
 		require.NoError(t, err)
 		require.Equal(t, nodeID, relogged.ID())

hscontrol/state/state.go

@@ -50,11 +50,14 @@ const (
 	// registerCacheExpiration defines how long node registration entries remain in cache.
 	registerCacheExpiration = time.Minute * 15
 
-	// defaultRegisterCacheMaxEntries is the default upper bound on the number
-	// of pending registration entries the auth cache will hold. Entries keep
-	// only hostname, OS and requested tags, capped by [types.ValidateHostinfo].
+	// defaultRegisterCacheMaxEntries bounds pending authentication transactions.
+	// Admission refuses new work at capacity instead of invalidating approvals.
 	defaultRegisterCacheMaxEntries = 1024
 
+	// authCacheSSHReserve keeps registered nodes' SSH checks available when
+	// unauthenticated registrations have filled their allocation.
+	authCacheSSHReserve = 64
+
 	// defaultNodeStoreBatchSize is the default number of write operations to batch
 	// before rebuilding the in-memory node snapshot.
 	defaultNodeStoreBatchSize = 100
@@ -126,6 +129,18 @@ var nodeUpdateColumns = []string{
 // ErrRegistrationExpired is returned when a registration has expired.
 var ErrRegistrationExpired = errors.New("registration expired")
 
+// ErrAuthCacheFull is returned when pending authentication has reached capacity.
+var ErrAuthCacheFull = errors.New("authentication capacity reached; retry later")
+
+// ErrAuthCacheEntryExists prevents replacing an existing authentication binding.
+var ErrAuthCacheEntryExists = errors.New("authentication request already exists")
+
+// ErrAuthCacheClosed prevents admitting authentication after shutdown.
+var ErrAuthCacheClosed = errors.New("authentication cache closed")
+
+// ErrAuthRequestFinished prevents reusing an already resolved verdict channel.
+var ErrAuthRequestFinished = errors.New("authentication request already finished")
+
 // ErrNodeKeyInUse is returned when a registration or re-auth claims a NodeKey
 // already bound to a different machine, enforcing the 1:1 NodeKey<->MachineKey
 // binding.
@@ -167,12 +182,14 @@ type State struct {
 	// polMan handles policy evaluation and management
 	polMan policy.PolicyManager
 
-	// authCache holds any pending authentication requests from either auth
-	// type (Web and OIDC). It is a bounded LRU keyed by AuthID; oldest
-	// entries are evicted once the size cap is reached, and entries that
-	// time out have their auth verdict resolved with ErrRegistrationExpired
-	// via the eviction callback so any waiting goroutines wake.
-	authCache *expirable.LRU[types.AuthID, *types.AuthRequest]
+	// authCache holds pending Web, OIDC and SSH authentication requests.
+	// Admission preserves unexpired entries at capacity. Expiry resolves the
+	// verdict with ErrRegistrationExpired so waiting goroutines wake.
+	authCache               *expirable.LRU[types.AuthID, *types.AuthRequest]
+	authCacheMu             sync.Mutex
+	authCacheMaxEntries     int
+	authCacheClosed         bool
+	authCacheRegularEntries atomic.Int64
 
 	// pings tracks pending ping requests and their response channels.
 	pings *pingTracker
@@ -353,29 +370,33 @@ func NewState(cfg *types.Config) (*State, error) {
 	nodeStore := NewNodeStore(nodes, policyPeersFunc(polMan), batchSize, batchTimeout)
 	nodeStore.Start()
 
-	// Last: NewLRU starts the TTL sweeper goroutine, which only Close stops.
-	authCache := expirable.NewLRU[types.AuthID, *types.AuthRequest](
-		cacheMaxEntries,
-		func(id types.AuthID, rn *types.AuthRequest) {
-			rn.FinishAuth(types.AuthVerdict{Err: ErrRegistrationExpired})
-		},
-		cacheExpiration,
-	)
-
 	s := &State{
 		cfg: cfg,
 
-		db:        db,
-		ipAlloc:   ipAlloc,
-		polMan:    polMan,
-		authCache: authCache,
-		nodeStore: nodeStore,
-		pings:     newPingTracker(),
+		db:                  db,
+		ipAlloc:             ipAlloc,
+		polMan:              polMan,
+		authCacheMaxEntries: cacheMaxEntries,
+		nodeStore:           nodeStore,
+		pings:               newPingTracker(),
 
 		sshCheckAuth:  make(map[sshCheckPair]time.Time),
 		registerLocks: xsync.NewMap[key.MachinePublic, *registerLock](),
 	}
 
+	// Last: NewLRU starts the TTL sweeper goroutine, which only Close stops.
+	s.authCache = expirable.NewLRU[types.AuthID, *types.AuthRequest](
+		cacheMaxEntries+authCacheSSHReserve,
+		func(_ types.AuthID, rn *types.AuthRequest) {
+			if !rn.IsSSHCheck() {
+				s.authCacheRegularEntries.Add(-1)
+			}
+
+			rn.FinishAuth(types.AuthVerdict{Err: ErrRegistrationExpired})
+		},
+		cacheExpiration,
+	)
+
 	// Surface nodes whose stored data would break map generation (e.g. an
 	// invalid given name from a legacy row) so an operator can fix them. This
 	// only logs; it never mutates a node's stored name at boot.
@@ -411,6 +432,10 @@ func policyPeersFunc(pm policy.PolicyManager) PeersFunc {
 
 // Close gracefully shuts down the [State] instance and releases all resources.
 func (s *State) Close() error {
+	s.authCacheMu.Lock()
+	s.authCacheClosed = true
+	s.authCacheMu.Unlock()
+
 	s.pings.drain()
 	s.nodeStore.Stop()
 	// Purge before Close: Close stops the sweeper, so anything still cached
@@ -1931,12 +1956,71 @@ func (s *State) DeleteExpiredAccessTokens(cutoff time.Time) (int64, error) {
 
 // GetAuthCacheEntry retrieves a pending auth request from the cache.
 func (s *State) GetAuthCacheEntry(id types.AuthID) (*types.AuthRequest, bool) {
-	return s.authCache.Get(id)
+	// Preserve insertion order so admission can reclaim expired entries from
+	// the oldest end without scanning every pending authentication.
+	return s.authCache.Peek(id)
 }
 
-// SetAuthCacheEntry stores a pending auth request in the cache.
-func (s *State) SetAuthCacheEntry(id types.AuthID, entry *types.AuthRequest) {
+// SetAuthCacheEntry admits a pending authentication without displacing existing
+// transactions. Re-admitting the same request does not extend its lifetime.
+func (s *State) SetAuthCacheEntry(id types.AuthID, entry *types.AuthRequest) error {
+	s.authCacheMu.Lock()
+	defer s.authCacheMu.Unlock()
+
+	if s.authCacheClosed {
+		return ErrAuthCacheClosed
+	}
+
+	if existing, ok := s.authCache.Peek(id); ok {
+		if existing == entry {
+			return nil
+		}
+
+		return ErrAuthCacheEntryExists
+	}
+
+	// Remove an expired entry with this ID before Add can overwrite its waiter.
+	s.authCache.Remove(id)
+
+	if entry.IsFinished() {
+		return ErrAuthRequestFinished
+	}
+
+	for {
+		oldest, _, ok := s.authCache.GetOldest()
+		if !ok {
+			break
+		}
+
+		if _, live := s.authCache.Peek(oldest); live {
+			break
+		}
+
+		s.authCache.Remove(oldest)
+	}
+
+	if (!entry.IsSSHCheck() && s.authCacheRegularEntries.Load() >= int64(s.authCacheMaxEntries)) ||
+		s.authCache.Len() >= s.authCacheMaxEntries+authCacheSSHReserve {
+		return ErrAuthCacheFull
+	}
+
+	err := entry.PrepareForCache()
+	if err != nil {
+		return err
+	}
+
+	if !entry.IsSSHCheck() {
+		s.authCacheRegularEntries.Add(1)
+	}
+
 	s.authCache.Add(id, entry)
+
+	return nil
+}
+
+// DeleteAuthCacheEntry releases a consumed authentication transaction.
+func (s *State) DeleteAuthCacheEntry(id types.AuthID) {
+	s.authCache.Remove(id)
 }
 
 // DeleteAuthCacheEntryForTest drops a pending auth request from the cache,
@@ -1944,7 +2028,7 @@ func (s *State) SetAuthCacheEntry(id types.AuthID, entry *types.AuthRequest) {
 // (expired, evicted, or dropped on a control-plane restart) without faking an
 // auth_id.
 func (s *State) DeleteAuthCacheEntryForTest(id types.AuthID) {
-	s.authCache.Remove(id)
+	s.DeleteAuthCacheEntry(id)
 }
 
 // SetLastSSHAuth records a successful SSH check authentication

hscontrol/types/common.go

@@ -4,6 +4,7 @@ package types
 //go:generate go run tailscale.com/cmd/viewer --type=User,Node,PreAuthKey,Credential
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"runtime"
@@ -22,9 +23,10 @@ const (
 
 // Common errors.
 var (
-	ErrCannotParsePrefix   = errors.New("cannot parse prefix")
-	ErrInvalidAuthIDLength = errors.New("auth ID has invalid length")
-	ErrInvalidAuthIDPrefix = errors.New("auth ID has invalid prefix")
+	ErrCannotParsePrefix           = errors.New("cannot parse prefix")
+	ErrInvalidAuthIDLength         = errors.New("auth ID has invalid length")
+	ErrInvalidAuthIDPrefix         = errors.New("auth ID has invalid prefix")
+	ErrRegistrationPayloadTooLarge = errors.New("registration payload exceeds size limit")
 )
 
 const (
@@ -109,7 +111,7 @@ type PendingRegistrationConfirmation struct {
 
 // AuthRequest represents a pending authentication request from a user or a
 // node. It carries the minimum data needed to either complete a node
-// registration (regData populated) or an SSH check-mode auth (sshBinding
+// registration (regData or regDataJSON populated) or an SSH check-mode auth (sshBinding
 // populated), and signals the verdict via the finished channel. The closed
 // flag guards [AuthRequest.FinishAuth] against double-close.
 //
@@ -117,14 +119,14 @@ type PendingRegistrationConfirmation struct {
 // have a single canonical instance even when stored in caches that
 // internally copy values.
 type AuthRequest struct {
-	// regData is populated for node-registration flows (interactive web
-	// or OIDC). It carries the cached registration payload that the
-	// auth callback uses to promote this request into a real node.
-	//
-	// nil for non-registration flows. Use [AuthRequest.RegistrationData] to read it
-	// safely.
+	// regData holds the caller's registration payload until PrepareForCache
+	// replaces it with owned serialized data and releases the original pointer.
 	regData *RegistrationData
 
+	// regDataJSON is immutable and bounded by MaxRegistrationPayloadBytes.
+	// RegistrationData decodes a fresh copy without retaining the decoded graph.
+	regDataJSON string
+
 	// sshBinding is populated for SSH check-mode flows. It captures the
 	// (src, dst) node pair the request was minted for so the follow-up
 	// and OIDC callback can refuse to record a verdict for any other
@@ -160,7 +162,8 @@ func NewAuthRequest() *AuthRequest {
 
 // NewRegisterAuthRequest creates a pending auth request carrying the
 // minimal [RegistrationData] for a node-registration flow. The data is
-// stored by pointer; callers must not mutate it after handing it off.
+// stored by pointer until [AuthRequest.PrepareForCache] detaches it; callers
+// must not mutate it until preparation finishes.
 func NewRegisterAuthRequest(data *RegistrationData) *AuthRequest {
 	return &AuthRequest{
 		regData:  data,
@@ -169,6 +172,31 @@ func NewRegisterAuthRequest(data *RegistrationData) *AuthRequest {
 	}
 }
 
+// PrepareForCache replaces the registration payload with an owned serialized
+// value of at most [MaxRegistrationPayloadBytes]. It must run before publishing
+// the request to the cache. Serialization and size errors leave it unchanged;
+// callers must reject admission on error. Repeated calls and non-registration
+// requests are no-ops.
+func (rn *AuthRequest) PrepareForCache() error {
+	if rn.regData == nil {
+		return nil
+	}
+
+	data, err := json.Marshal(rn.regData)
+	if err != nil {
+		return fmt.Errorf("serializing pending registration: %w", err)
+	}
+
+	if len(data) > MaxRegistrationPayloadBytes {
+		return fmt.Errorf("%w: limit %d bytes", ErrRegistrationPayloadTooLarge, MaxRegistrationPayloadBytes)
+	}
+
+	rn.regDataJSON = string(data)
+	rn.regData = nil
+
+	return nil
+}
+
 // NewSSHCheckAuthRequest creates a pending auth request bound to a
 // specific (src, dst) SSH check-mode pair. The follow-up handler and
 // OIDC callback must verify their incoming request matches this binding
@@ -184,15 +212,27 @@ func NewSSHCheckAuthRequest(src, dst NodeID) *AuthRequest {
 	}
 }
 
-// RegistrationData returns the cached registration payload. It panics if
+// RegistrationData returns the original registration payload before preparation,
+// or a fresh decoded copy after [AuthRequest.PrepareForCache]. It panics if
 // called on an [AuthRequest] that was not created via
 // [NewRegisterAuthRequest].
 func (rn *AuthRequest) RegistrationData() *RegistrationData {
-	if rn.regData == nil {
+	if rn.regData != nil {
+		return rn.regData
+	}
+
+	if rn.regDataJSON == "" {
 		panic("RegistrationData can only be used in registration requests")
 	}
 
-	return rn.regData
+	var data RegistrationData
+
+	err := json.Unmarshal([]byte(rn.regDataJSON), &data)
+	if err != nil {
+		panic(fmt.Sprintf("decoding prepared registration: %v", err))
+	}
+
+	return &data
 }
 
 // SSHCheckBinding returns the (src, dst) node pair an SSH check-mode
@@ -209,7 +249,7 @@ func (rn *AuthRequest) SSHCheckBinding() *SSHCheckBinding {
 // IsRegistration reports whether this auth request carries registration
 // data (i.e. it was created via [NewRegisterAuthRequest]).
 func (rn *AuthRequest) IsRegistration() bool {
-	return rn.regData != nil
+	return rn.regData != nil || rn.regDataJSON != ""
 }
 
 // IsSSHCheck reports whether this auth request is bound to an SSH
@@ -219,6 +259,11 @@ func (rn *AuthRequest) IsSSHCheck() bool {
 	return rn.sshBinding != nil
 }
 
+// IsFinished reports whether a verdict has already resolved this request.
+func (rn *AuthRequest) IsFinished() bool {
+	return rn.closed.Load()
+}
+
 // SetPendingConfirmation marks this [AuthRequest] as having an
 // OIDC-resolved user that is waiting to confirm the registration on
 // the interstitial. The OIDC callback should call this and then render

hscontrol/types/common_test.go

@@ -1,10 +1,18 @@
 package types
 
 import (
+	"encoding/json"
+	"fmt"
+	"math"
+	"net/netip"
+	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/key"
 )
 
 // TestNewSSHCheckAuthRequestBinding verifies that an SSH-check [AuthRequest]
@@ -44,6 +52,195 @@ func TestNewRegisterAuthRequestPayload(t *testing.T) {
 	}, "SSHCheckBinding() must panic on a registration AuthRequest")
 }
 
+func TestAuthRequestPrepareForCacheOwnsRegistration(t *testing.T) {
+	expiry := time.Date(2027, time.January, 2, 3, 4, 5, 6, time.UTC)
+	data := &RegistrationData{
+		MachineKey: key.NewMachine().Public(),
+		NodeKey:    key.NewNode().Public(),
+		DiscoKey:   key.NewDisco().Public(),
+		Hostname:   "router-a",
+		Hostinfo: &tailcfg.Hostinfo{
+			Hostname:     "router-a",
+			IPNVersion:   "1.104.0",
+			OS:           "linux",
+			OSVersion:    "6.18.0",
+			Distro:       "nixos",
+			Container:    "false",
+			Userspace:    "true",
+			RequestTags:  []string{"tag:router", "tag:production"},
+			RoutableIPs:  []netip.Prefix{netip.MustParsePrefix("10.0.0.0/24"), netip.MustParsePrefix("2001:db8::/64")},
+			WoLMACs:      []string{"00:11:22:33:44:55"},
+			Services:     []tailcfg.Service{{Proto: tailcfg.TCP, Port: 22, Description: "ssh"}},
+			SSH_HostKeys: []string{"ssh-ed25519 initial-host-key"},
+			NetInfo: &tailcfg.NetInfo{
+				WorkingUDP:    "true",
+				WorkingIPv6:   "false",
+				PreferredDERP: 1,
+				DERPLatency:   map[string]float64{"1-v4": 0.01, "2-v6": 0.02},
+			},
+			Location: &tailcfg.Location{Country: "Norway", City: "Tromsø", Latitude: 69.65, Longitude: 18.96},
+			TPM:      &tailcfg.TPMInfo{Manufacturer: "MSFT", Model: 1},
+		},
+		Endpoints: []netip.AddrPort{netip.MustParseAddrPort("192.0.2.1:41641"), netip.MustParseAddrPort("[2001:db8::1]:41641")},
+		Expiry:    &expiry,
+	}
+	want := *data
+	want.Hostinfo = data.Hostinfo.Clone()
+	want.Endpoints = append([]netip.AddrPort(nil), data.Endpoints...)
+	wantExpiry := expiry
+	want.Expiry = &wantExpiry
+
+	req := NewRegisterAuthRequest(data)
+	require.NoError(t, req.PrepareForCache())
+	assert.True(t, req.IsRegistration())
+	assert.False(t, req.IsSSHCheck())
+	assert.Nil(t, req.regData, "prepared requests must release the decoded payload")
+	assert.Equal(t, &want, req.RegistrationData())
+
+	data.Hostname = "changed"
+	data.Hostinfo.RequestTags[0] = "tag:changed"
+	data.Hostinfo.RoutableIPs[0] = netip.MustParsePrefix("192.0.2.0/24")
+	data.Hostinfo.Services[0].Description = "changed"
+	data.Hostinfo.NetInfo.DERPLatency["1-v4"] = 1
+	data.Hostinfo.Location.City = "changed"
+	data.Hostinfo.TPM.Manufacturer = "changed"
+	data.Endpoints[0] = netip.MustParseAddrPort("192.0.2.2:1")
+	*data.Expiry = time.Time{}
+
+	assert.Equal(t, &want, req.RegistrationData(), "the producer must not retain ownership of cached data")
+
+	got := req.RegistrationData()
+	got.Hostname = "changed by consumer"
+	got.Hostinfo.RequestTags[0] = "tag:consumer"
+	got.Hostinfo.Services[0].Description = "changed by consumer"
+	got.Hostinfo.NetInfo.DERPLatency["1-v4"] = 2
+	got.Endpoints[0] = netip.MustParseAddrPort("192.0.2.3:1")
+	*got.Expiry = time.Time{}
+
+	require.NoError(t, req.PrepareForCache())
+	assert.Equal(t, &want, req.RegistrationData(), "access and repeated preparation must leave cached data unchanged")
+}
+
+func TestAuthRequestPrepareForCachePayloadLimit(t *testing.T) {
+	const limit = MaxRegistrationPayloadBytes
+
+	for _, test := range []struct {
+		name string
+		size int
+	}{
+		{name: "below limit", size: limit - 1},
+		{name: "exact limit", size: limit},
+		{name: "over limit", size: limit + 1},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			data := &RegistrationData{Hostinfo: &tailcfg.Hostinfo{OS: "linux"}}
+			base, err := json.Marshal(data)
+			require.NoError(t, err)
+
+			data.Hostinfo.OS = strings.Repeat("a", test.size-len(base)+len(data.Hostinfo.OS))
+			encoded, err := json.Marshal(data)
+			require.NoError(t, err)
+			require.Len(t, encoded, test.size)
+
+			req := NewRegisterAuthRequest(data)
+
+			err = req.PrepareForCache()
+			if test.size > limit {
+				require.ErrorIs(t, err, ErrRegistrationPayloadTooLarge, "oversize payloads must be rejected instead of truncated")
+				assert.Empty(t, req.regDataJSON)
+				assert.Same(t, data, req.RegistrationData(), "failed preparation must leave the original data intact")
+
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Nil(t, req.regData)
+			assert.Len(t, req.regDataJSON, test.size)
+			assert.Equal(t, data, req.RegistrationData())
+		})
+	}
+}
+
+func TestAuthRequestPrepareForCacheRouteAndServiceMetadata(t *testing.T) {
+	data := &RegistrationData{
+		MachineKey: key.NewMachine().Public(),
+		NodeKey:    key.NewNode().Public(),
+		Hostname:   "regional-router",
+		Hostinfo: &tailcfg.Hostinfo{
+			Hostname:    "regional-router",
+			OS:          "linux",
+			OSVersion:   "6.18.0",
+			IPNVersion:  "1.104.0",
+			RequestTags: []string{"tag:router", "tag:production"},
+		},
+	}
+	for i := range 128 {
+		data.Hostinfo.RoutableIPs = append(data.Hostinfo.RoutableIPs,
+			netip.MustParsePrefix(fmt.Sprintf("10.42.%d.0/24", i)),
+			netip.MustParsePrefix(fmt.Sprintf("2001:db8:%x::/64", i)))
+	}
+
+	for port := uint16(8000); port < 8064; port++ {
+		data.Hostinfo.Services = append(data.Hostinfo.Services, tailcfg.Service{
+			Proto:       tailcfg.TCP,
+			Port:        port,
+			Description: fmt.Sprintf("regional-service-%d", port),
+		})
+	}
+
+	req := NewRegisterAuthRequest(data)
+	require.NoError(t, req.PrepareForCache())
+	assert.Nil(t, req.regData)
+	assert.LessOrEqual(t, len(req.regDataJSON), MaxRegistrationPayloadBytes)
+	assert.Equal(t, data, req.RegistrationData())
+}
+
+func TestAuthRequestPrepareForCacheInvalidPayload(t *testing.T) {
+	invalidExpiry := time.Date(10000, time.January, 1, 0, 0, 0, 0, time.UTC)
+	for _, test := range []struct {
+		name string
+		data *RegistrationData
+	}{
+		{name: "invalid expiry", data: &RegistrationData{Expiry: &invalidExpiry}},
+		{name: "invalid latency", data: &RegistrationData{Hostinfo: &tailcfg.Hostinfo{
+			NetInfo: &tailcfg.NetInfo{DERPLatency: map[string]float64{"1-v4": math.NaN()}},
+		}}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			req := NewRegisterAuthRequest(test.data)
+			require.Error(t, req.PrepareForCache())
+			assert.Empty(t, req.regDataJSON)
+			assert.Same(t, test.data, req.RegistrationData())
+		})
+	}
+}
+
+func TestAuthRequestPrepareForCacheNilValues(t *testing.T) {
+	for _, data := range []*RegistrationData{
+		{},
+		{Hostinfo: &tailcfg.Hostinfo{}},
+		{Hostinfo: &tailcfg.Hostinfo{NetInfo: &tailcfg.NetInfo{}}},
+		{Expiry: &time.Time{}},
+		{Endpoints: []netip.AddrPort{}},
+	} {
+		req := NewRegisterAuthRequest(data)
+		require.NoError(t, req.PrepareForCache())
+		assert.True(t, req.IsRegistration())
+		assert.Equal(t, data, req.RegistrationData())
+	}
+
+	for _, req := range []*AuthRequest{NewAuthRequest(), NewRegisterAuthRequest(nil), NewSSHCheckAuthRequest(7, 11)} {
+		require.NoError(t, req.PrepareForCache())
+		require.NoError(t, req.PrepareForCache())
+		assert.False(t, req.IsRegistration())
+		assert.Panics(t, func() { _ = req.RegistrationData() })
+
+		if req.IsSSHCheck() {
+			assert.Equal(t, &SSHCheckBinding{SrcNodeID: 7, DstNodeID: 11}, req.SSHCheckBinding())
+		}
+	}
+}
+
 // TestNewAuthRequestEmptyPayload verifies that a payload-less
 // [AuthRequest] reports both Is* helpers as false and panics on either
 // payload accessor.

hscontrol/types/config.go

@@ -462,9 +462,9 @@ type Tuning struct {
 	// valid before being eligible for eviction.
 	RegisterCacheExpiration time.Duration
 
-	// RegisterCacheMaxEntries bounds the number of pending registration
-	// entries the auth cache will hold. Older entries are evicted (LRU)
-	// when the cap is reached, preventing unauthenticated cache-fill DoS.
+	// RegisterCacheMaxEntries bounds pending registrations. At capacity, new
+	// registrations are refused without evicting admitted sessions. SSH checks
+	// can also use 64 reserved slots beyond this limit.
 	// A value of 0 falls back to defaultRegisterCacheMaxEntries (1024).
 	RegisterCacheMaxEntries int
 

hscontrol/types/registration.go

@@ -8,14 +8,25 @@ import (
 	"tailscale.com/types/key"
 )
 
+// MaxRegistrationPayloadBytes limits the owned serialized registration payload
+// retained by an [AuthRequest] after [AuthRequest.PrepareForCache]. It covers
+// the authentication metadata and the minimal registration Hostinfo.
+// It bounds retained serialized bytes, not transient encoding or decoded heap
+// usage. A cache of 1024 entries retains at most 64 MiB of these payloads plus
+// record overhead; a different entry limit scales that budget.
+const MaxRegistrationPayloadBytes = 64 << 10
+
 // RegistrationData is the payload cached for a pending node registration.
 // It replaces the previous practice of caching a full *[Node] and carries
 // only the fields the registration callback path actually consumes when
 // promoting a pending registration to a real node.
 //
-// Any client can open a pending registration. The bounded-LRU cache that
-// holds these entries caps their number. Hostinfo retains only auth and
+// Any client can open a pending registration. Cache admission caps the number
+// of pending entries. Hostinfo retains only auth and
 // confirmation metadata, bounded by [ValidateHostinfo] before caching.
+//
+// [AuthRequest.PrepareForCache] retains an owned serialized representation
+// bounded by [MaxRegistrationPayloadBytes], releasing this decoded payload.
 type RegistrationData struct {
 	// MachineKey is the cryptographic identity of the machine being
 	// registered. Required.