api/v2: add OAuth client-credentials auth and scope enforcement

hscontrol/api/v2/README.md

@@ -1,7 +1,7 @@
-# API v2 — Headscale's v2 API
+# API v2: Headscale's v2 API
 
 This is Headscale's v2 HTTP API, served at `/api/v2`. Some of its endpoints are
-**ported from Tailscale's API** — reusing Tailscale's wire shapes — so the
+**ported from Tailscale's API**, reusing Tailscale's wire shapes, so the
 Tailscale ecosystem that cannot talk to Headscale today works: the
 [Terraform/OpenTofu provider], [tscli], and the official [Go client]
 (`tailscale.com/client/tailscale/v2`).
@@ -23,12 +23,20 @@ headscale's own conventions. The headscale-native admin API stays at `/api/v1`
 - Errors use **Tailscale's** body (`{"message","data","status"}`), installed as
   a per-API transform (`tailscaleErrorTransformer` in `errors.go`). A future
   headscale-native v2 operation would keep Huma's RFC 9457 problem+json.
-- Auth accepts the API key as **HTTP Basic** (key as username — what the SDK
-  sends) or **Bearer**. See `authMiddleware`.
-- Each operation declares the Tailscale scope it would require (`auth_keys`,
-  `devices:core`, `devices:routes`, `policy_file`, `feature_settings`, each with
-  a `:read` subset). Nothing is enforced yet — every key is all-access — pending
-  OAuth tokens; see the `TODO(scopes)` in `api.go`.
+- Auth accepts a credential as **HTTP Basic** (key as username, what the SDK
+  sends) or **Bearer**: an admin API key (`hskey-api-…`), or an OAuth access
+  token (`hskey-oauthtok-…`). See `authMiddleware`.
+- Each operation declares the Tailscale scope it requires (`auth_keys`,
+  `oauth_keys`, `devices:core`, `devices:routes`, `policy_file`,
+  `feature_settings`, each with a `:read` subset, plus `all`/`all:read`).
+  `requireScope` records it both for the middleware and in the generated
+  OpenAPI, as an `x-required-scope` extension and a sentence in the operation
+  description, so the scope shows up in the docs and spec. Enforcement: an
+  **admin API key is all-access** (scope checks skipped); an **OAuth access
+  token is scope-limited**, the middleware checks the operation's declared scope
+  against the token's grant (`scope.Grants`, where a write scope subsumes its
+  `:read` and `all`/`all:read` are super-scopes). The two are told apart by
+  credential prefix.
 - Resolve one entity by id with a typed getter (`GetNodeByID`, `GetUserByID`,
   `GetAPIKeyByID`, `GetPreAuthKeyByID`); add one to state/db if it is missing
   rather than scanning a `List`. Build responses from the view accessors
@@ -38,6 +46,35 @@ headscale's own conventions. The headscale-native admin API stays at `/api/v1`
   `ExpirySeconds *time.Duration` marshals as nanoseconds, which the spec and
   every client read as seconds.
 
+## OAuth clients & scopes
+
+Most of the Tailscale ecosystem (the Terraform provider, `tscli`, the Go client)
+accepts **either** an API key **or OAuth 2.0 client-credentials**; the Kubernetes
+operator is OAuth-only. Supporting OAuth lets all of them drive Headscale.
+
+- **OAuth clients** are not a separate resource; they are `keyType:"client"` on
+  the keys endpoint, exactly as Tailscale does it. Create
+  (`POST /api/v2/tailnet/-/keys` with `{"keyType":"client","scopes":[…],"tags":[…]}`)
+  returns a `Key` whose `id` is the client id and whose `key` is the secret,
+  **shown once**; get/list never re-expose it. The secret is
+  `hskey-client-<clientID>-<secret>`, embedding the client id so the token
+  endpoint derives it from the secret (Tailscale's `get-authkey` trick). See
+  `keys.go` (`createOAuthClient`) and `db/oauth.go`.
+- **Token endpoint** `POST /api/v2/oauth/token` (`oauth.go`) is a plain handler,
+  not a Huma operation: it takes `application/x-www-form-urlencoded` and emits
+  RFC 6749 OAuth2 error bodies (`{"error","error_description"}`). Credentials
+  arrive in the body or HTTP Basic; optional space-delimited `scope`/`tags`
+  narrow the token to a subset of the client's grant. It returns a 1-hour
+  `Bearer` access token (`hskey-oauthtok-…`).
+- **Scope enforcement** is the one seam in `authMiddleware`. **Tag enforcement**:
+  an auth key minted by a token may only carry tags the token holds, or tags
+  owned-by them via the policy `tagOwners` (`State.TagOwnedByTags` →
+  `policy/v2`), so e.g. an operator token tagged `tag:k8s-operator` may mint
+  `tag:k8s` keys.
+- Credentials/tokens are stored like API keys: a public id/prefix plus an
+  **Argon2id** hash of the secret (no JWT, no signing keys). `OAuthClient` and
+  `OAuthAccessToken` live in `types/oauth.go` and `db/oauth.go`.
+
 ## Adding an endpoint
 
 Worked example: the keys resource (`keys.go`) = Tailscale auth keys = Headscale
@@ -55,7 +92,7 @@ pre-auth keys.
 
 3. **Map to Headscale.** Write the field ↔ field ↔ `state` call mapping. Record
    gaps and the decision for each (e.g. Tailscale `preauthorized` has no
-   Headscale equivalent — accepted, ignored, echoed back). _Acceptance: every
+   Headscale equivalent: accepted, ignored, echoed back). _Acceptance: every
    request field is consumed or deliberately ignored; every response field has a
    source._
 
@@ -73,13 +110,13 @@ compat`; declare its `Errors`; enforce the tailnet and scope. Map state
 
 6. **Roundtrip the real clients.** Add a `t.Run` subtest to `TestAPIv2`
    (`hscontrol/servertest/apiv2_test.go`) for each of the Go client, tscli, and
-   OpenTofu — full create→read→list→delete against one shared server on a real
+   OpenTofu, full create→read→list→delete against one shared server on a real
    loopback port (`servertest.WithRealListener`). tscli and tofu come from the
    nix dev shell; a missing binary fails the test. _Acceptance: `nix develop -c
 go test ./hscontrol/servertest/ -run TestAPIv2` is green._
 
 7. **Update the CLI** only if the v2 operation fully replaces a v1 one. Tailscale
-   has no separate key-expire verb — its `DELETE` _is_ the revoke — so v2 maps
+   has no separate key-expire verb (its `DELETE` _is_ the revoke), so v2 maps
    `DELETE` to a soft revoke: the key stays retrievable with `invalid: true`
    until the collector reaps it (`preauth_keys.revoked_retention`), the
    equivalent of v1 `preauthkeys expire`. `headscale preauthkeys` still stays on

hscontrol/api/v2/acl.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"github.com/danielgtaylor/huma/v2"
+	"github.com/juanfont/headscale/hscontrol/scope"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/juanfont/headscale/hscontrol/util"
 )
@@ -60,7 +61,7 @@ func registerACL(api huma.API, b Backend) {
 		Tags:        aclTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound, http.StatusInternalServerError},
-	}, ScopePolicyFileRead), func(ctx context.Context, in *getACLInput) (*huma.StreamResponse, error) {
+	}, scope.PolicyFileRead), func(ctx context.Context, in *getACLInput) (*huma.StreamResponse, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
@@ -89,7 +90,7 @@ func registerACL(api huma.API, b Backend) {
 			http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden,
 			http.StatusNotFound, http.StatusPreconditionFailed, http.StatusInternalServerError,
 		},
-	}, ScopePolicyFile), func(ctx context.Context, in *setACLInput) (*huma.StreamResponse, error) {
+	}, scope.PolicyFile), func(ctx context.Context, in *setACLInput) (*huma.StreamResponse, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err

hscontrol/api/v2/api.go

@@ -1,8 +1,8 @@
 // Package apiv2 is Headscale's v2 HTTP API, served at /api/v2.
 //
 // Where the v1 API (hscontrol/api/v1) is the headscale-native admin surface, v2
-// additionally ports selected endpoints from Tailscale's API — reusing
-// Tailscale's wire shapes (paths, request/response JSON, error body) — so the
+// additionally ports selected endpoints from Tailscale's API, reusing
+// Tailscale's wire shapes (paths, request/response JSON, error body), so the
 // existing Tailscale ecosystem (the Terraform/OpenTofu provider, tscli, and
 // tailscale.com/client/tailscale/v2) can drive Headscale unchanged. Ported
 // operations carry the "Tailscale compat" tag; a headscale-native v2 operation
@@ -24,6 +24,7 @@ import (
 	"github.com/danielgtaylor/huma/v2"
 	"github.com/danielgtaylor/huma/v2/adapters/humachi"
 	"github.com/go-chi/chi/v5"
+	"github.com/juanfont/headscale/hscontrol/scope"
 	"github.com/juanfont/headscale/hscontrol/state"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/juanfont/headscale/hscontrol/types/change"
@@ -40,7 +41,7 @@ type Backend struct {
 }
 
 // security is the requirement applied to authenticated operations: an API key
-// presented as HTTP Basic (the key as username — what the Tailscale SDK sends)
+// presented as HTTP Basic (the key as username, what the Tailscale SDK sends)
 // or as a Bearer token.
 var security = []map[string][]string{{"basicAuth": {}}, {"bearerAuth": {}}}
 
@@ -79,7 +80,7 @@ func Config() huma.Config {
 	// Accept application/hujson request bodies (the Tailscale SDK sends the
 	// policy file that way). The bytes are captured raw by the ACL handler, so
 	// reusing the JSON format is only to satisfy huma's content-type check.
-	// Clone first — config.Formats aliases huma's shared DefaultFormats map.
+	// Clone first: config.Formats aliases huma's shared DefaultFormats map.
 	formats := maps.Clone(config.Formats)
 	formats["application/hujson"] = formats["application/json"]
 	config.Formats = formats
@@ -98,6 +99,10 @@ func NewAPI(router chi.Router, backend Backend) huma.API {
 
 	Register(api, backend)
 
+	// The OAuth token endpoint is a plain route, not a Huma operation (see
+	// oauth.go); register it on the same router.
+	registerOAuthToken(router, backend)
+
 	return api
 }
 
@@ -117,11 +122,21 @@ func Spec() ([]byte, error) {
 	return api.OpenAPI().YAML()
 }
 
+// Spec30 emits the document downgraded to OpenAPI 3.0.3, needed because
+// oapi-codegen cannot yet read 3.1; the typed client is generated from this.
+func Spec30() ([]byte, error) {
+	api := NewAPI(chi.NewMux(), Backend{})
+
+	return api.OpenAPI().DowngradeYAML()
+}
+
 type contextKey int
 
 const (
 	localTrustKey contextKey = iota
 	ownerUserKey
+	principalScopesKey
+	principalTagsKey
 )
 
 // WithLocalTrust marks a request as arriving over a locally-trusted transport
@@ -136,8 +151,8 @@ func WithLocalTrust(next http.Handler) http.Handler {
 }
 
 // authMiddleware authenticates the API key (HTTP Basic with the key as the
-// username — what the Tailscale SDK sends — or Bearer), records the key's
-// owning user for handlers, and enforces the operation's required scope.
+// username, what the Tailscale SDK sends, or Bearer), records the key's owning
+// user for handlers, and enforces the operation's required scope.
 func authMiddleware(api huma.API, b Backend) func(huma.Context, func(huma.Context)) {
 	return func(ctx huma.Context, next func(huma.Context)) {
 		if ctx.Context().Value(localTrustKey) != nil {
@@ -159,6 +174,35 @@ func authMiddleware(api huma.API, b Backend) func(huma.Context, func(huma.Contex
 			return
 		}
 
+		// An OAuth access token is scope-limited; an admin API key is all-access.
+		// They are told apart by prefix so a scoped token can never be mistaken
+		// for an all-access key.
+		if strings.HasPrefix(token, types.AccessTokenPrefix) {
+			at, err := b.State.AuthenticateAccessToken(token)
+			if err != nil {
+				_ = huma.WriteErr(api, ctx, http.StatusUnauthorized, "unauthorized")
+
+				return
+			}
+
+			if want, ok := requiredScope(ctx.Operation()); ok && !scope.Grants(scope.Parse(at.Scopes), want) {
+				_ = huma.WriteErr(api, ctx, http.StatusForbidden,
+					"token is missing the required scope "+string(want))
+
+				return
+			}
+
+			// The keys handler multiplexes on keyType, so its required scope and
+			// permitted tags depend on the body; carry the token's scopes and tags
+			// for it to finish the check the static middleware cannot.
+			ctx = huma.WithValue(ctx, principalScopesKey, at.Scopes)
+			ctx = huma.WithValue(ctx, principalTagsKey, at.Tags)
+
+			next(ctx)
+
+			return
+		}
+
 		key, err := b.State.AuthenticateAPIKey(token)
 		if err != nil {
 			_ = huma.WriteErr(api, ctx, http.StatusUnauthorized, "unauthorized")
@@ -166,13 +210,9 @@ func authMiddleware(api huma.API, b Backend) func(huma.Context, func(huma.Contex
 			return
 		}
 
-		// TODO(scopes): every valid key is all-access today. Operations still
-		// declare their required scope (requireScope) so that once OAuth tokens
-		// arrive, the granted set can be derived from the token and checked
-		// against the operation's scope here. Until then, accept everything.
-
-		// Record the key's owning user (may be unset) so handlers can create
-		// user-owned keys on its behalf.
+		// An admin API key is all-access: its operations are not scope-checked.
+		// Record its owning user (may be unset) so handlers can create user-owned
+		// keys on its behalf.
 		if key.UserID != nil {
 			ctx = huma.WithValue(ctx, ownerUserKey, types.UserID(*key.UserID))
 		}
@@ -210,6 +250,24 @@ func ownerUser(ctx context.Context) (types.UserID, bool) {
 	return uid, ok
 }
 
+// principalScopes returns the scopes granted to the request's OAuth access
+// token, and whether the request authenticated with one. ok is false for an
+// admin API key, which is all-access and not scope-checked.
+func principalScopes(ctx context.Context) ([]string, bool) {
+	scopes, ok := ctx.Value(principalScopesKey).([]string)
+
+	return scopes, ok
+}
+
+// principalTags returns the tags granted to the request's OAuth access token,
+// and whether the request authenticated with one. An admin API key is not an
+// OAuth token, so ok is false and its key creation is unrestricted by tags.
+func principalTags(ctx context.Context) ([]string, bool) {
+	tags, ok := ctx.Value(principalTagsKey).([]string)
+
+	return tags, ok
+}
+
 // requireDefaultTailnet rejects any tailnet other than "-". Headscale is
 // single-tailnet; the Tailscale SDK sends "-" (its default tailnet). A non-"-"
 // value is "no such tailnet", a 404, which lets the SDK's IsNotFound behave.
@@ -221,46 +279,47 @@ func requireDefaultTailnet(tailnet string) error {
 	return nil
 }
 
-// Scope is an OAuth capability an operation requires and a token grants. The
-// names mirror Tailscale's API scopes (see the OAuth scope descriptions in the
-// Tailscale OpenAPI spec); a ...Read scope is the read-only subset of its
-// write scope. Nothing is enforced yet — every key is all-access — but every
-// operation declares the scope it would require, so OAuth tokens can later be
-// checked against it without reworking the operations.
-type Scope string
+// The scope vocabulary and the grant predicate live in the hscontrol/scope
+// package; this file only wires a required scope onto each huma operation and
+// reads it back in the middleware.
 
-const (
-	ScopeAuthKeys     Scope = "auth_keys"
-	ScopeAuthKeysRead Scope = "auth_keys:read"
-
-	ScopeDevicesCore     Scope = "devices:core"
-	ScopeDevicesCoreRead Scope = "devices:core:read"
-
-	ScopeDevicesRoutes     Scope = "devices:routes"
-	ScopeDevicesRoutesRead Scope = "devices:routes:read"
-
-	ScopePolicyFile     Scope = "policy_file"
-	ScopePolicyFileRead Scope = "policy_file:read"
-
-	ScopeFeatureSettings     Scope = "feature_settings"
-	ScopeFeatureSettingsRead Scope = "feature_settings:read"
-
-	ScopeUsers     Scope = "users"
-	ScopeUsersRead Scope = "users:read"
-)
-
-// scopeMetaKey keys the per-operation required Scope in huma.Operation.Metadata.
+// scopeMetaKey keys the per-operation required scope in huma.Operation.Metadata.
 const scopeMetaKey = "headscale.scope"
 
-// requireScope records op's required scope in its Metadata, where the auth
-// middleware can read it back. It keeps the requirement next to the operation
-// definition.
-func requireScope(op huma.Operation, s Scope) huma.Operation {
+// requireScope records op's required scope, both in its Metadata (where the auth
+// middleware reads it back) and in the generated OpenAPI document: an
+// x-required-scope extension for machine consumers and a Description line so the
+// rendered docs state what each operation needs.
+func requireScope(op huma.Operation, s scope.Scope) huma.Operation {
 	if op.Metadata == nil {
 		op.Metadata = map[string]any{}
 	}
 
 	op.Metadata[scopeMetaKey] = s
 
+	if op.Extensions == nil {
+		op.Extensions = map[string]any{}
+	}
+
+	op.Extensions["x-required-scope"] = string(s)
+
+	note := "Requires the `" + string(s) + "` OAuth scope (an admin API key is all-access)."
+	if op.Description == "" {
+		op.Description = note
+	} else {
+		op.Description += "\n\n" + note
+	}
+
 	return op
 }
+
+// requiredScope returns the scope an operation declared via requireScope, if any.
+func requiredScope(op *huma.Operation) (scope.Scope, bool) {
+	if op == nil || op.Metadata == nil {
+		return "", false
+	}
+
+	s, ok := op.Metadata[scopeMetaKey].(scope.Scope)
+
+	return s, ok
+}

hscontrol/api/v2/devices.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/danielgtaylor/huma/v2"
+	"github.com/juanfont/headscale/hscontrol/scope"
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/juanfont/headscale/hscontrol/util"
 	"tailscale.com/net/tsaddr"
@@ -125,7 +126,7 @@ func registerDevices(api huma.API, b Backend) {
 		Tags:        deviceTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCoreRead), func(ctx context.Context, in *deviceByIDInput) (*deviceOutput, error) {
+	}, scope.DevicesCoreRead), func(ctx context.Context, in *deviceByIDInput) (*deviceOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -142,7 +143,7 @@ func registerDevices(api huma.API, b Backend) {
 		Tags:        deviceTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCoreRead), func(ctx context.Context, in *listDevicesInput) (*listDevicesOutput, error) {
+	}, scope.DevicesCoreRead), func(ctx context.Context, in *listDevicesInput) (*listDevicesOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
@@ -170,7 +171,7 @@ func registerDevices(api huma.API, b Backend) {
 		Security:      security,
 		DefaultStatus: http.StatusOK,
 		Errors:        []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCore), func(ctx context.Context, in *deviceByIDInput) (*emptyOutput, error) {
+	}, scope.DevicesCore), func(ctx context.Context, in *deviceByIDInput) (*emptyOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -195,7 +196,7 @@ func registerDevices(api huma.API, b Backend) {
 		Security:      security,
 		DefaultStatus: http.StatusOK,
 		Errors:        []int{http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCore), func(ctx context.Context, in *setAuthorizedInput) (*emptyOutput, error) {
+	}, scope.DevicesCore), func(ctx context.Context, in *setAuthorizedInput) (*emptyOutput, error) {
 		_, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -222,7 +223,7 @@ func registerDevices(api huma.API, b Backend) {
 		Security:      security,
 		DefaultStatus: http.StatusOK,
 		Errors:        []int{http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCore), func(ctx context.Context, in *setNameInput) (*emptyOutput, error) {
+	}, scope.DevicesCore), func(ctx context.Context, in *setNameInput) (*emptyOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -247,7 +248,7 @@ func registerDevices(api huma.API, b Backend) {
 		Security:      security,
 		DefaultStatus: http.StatusOK,
 		Errors:        []int{http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCore), func(ctx context.Context, in *setTagsInput) (*emptyOutput, error) {
+	}, scope.DevicesCore), func(ctx context.Context, in *setTagsInput) (*emptyOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -261,6 +262,19 @@ func registerDevices(api huma.API, b Backend) {
 			return &emptyOutput{}, nil
 		}
 
+		// An OAuth token may only assign tags within its grant (held directly or
+		// owned by a held tag per policy); an admin API key is unrestricted. The
+		// devices:core scope alone must not let a token stamp an arbitrary policy
+		// tag (e.g. tag:prod) onto any node. SetNodeTags still enforces that each
+		// tag exists in policy.
+		if tokenTags, isOAuth := principalTags(ctx); isOAuth {
+			for _, tag := range in.Body.Tags {
+				if !b.State.TagOwnedByTags(tag, tokenTags) {
+					return nil, huma.Error403Forbidden("token may not assign tag " + tag)
+				}
+			}
+		}
+
 		_, nodeChange, err := b.State.SetNodeTags(node.ID(), in.Body.Tags)
 		if err != nil {
 			return nil, mapError("setting device tags", err)
@@ -280,7 +294,7 @@ func registerDevices(api huma.API, b Backend) {
 		Security:      security,
 		DefaultStatus: http.StatusOK,
 		Errors:        []int{http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesCore), func(ctx context.Context, in *setKeyInput) (*emptyOutput, error) {
+	}, scope.DevicesCore), func(ctx context.Context, in *setKeyInput) (*emptyOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -313,7 +327,7 @@ func registerDevices(api huma.API, b Backend) {
 		Security:      security,
 		DefaultStatus: http.StatusOK,
 		Errors:        []int{http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesRoutes), func(ctx context.Context, in *setSubnetRoutesInput) (*deviceRoutesOutput, error) {
+	}, scope.DevicesRoutes), func(ctx context.Context, in *setSubnetRoutesInput) (*deviceRoutesOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err
@@ -342,7 +356,7 @@ func registerDevices(api huma.API, b Backend) {
 		Tags:        deviceTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeDevicesRoutesRead), func(ctx context.Context, in *deviceByIDInput) (*deviceRoutesOutput, error) {
+	}, scope.DevicesRoutesRead), func(ctx context.Context, in *deviceByIDInput) (*deviceRoutesOutput, error) {
 		node, err := lookupNode(b, in.DeviceID)
 		if err != nil {
 			return nil, err

hscontrol/api/v2/errors.go

@@ -9,12 +9,12 @@ import (
 	"gorm.io/gorm"
 )
 
-// apiError is the Tailscale API error body. The official Tailscale Go client —
-// and therefore the Terraform provider and tscli built on it — decode 4xx/5xx
+// apiError is the Tailscale API error body. The official Tailscale Go client
+// (and therefore the Terraform provider and tscli built on it) decodes 4xx/5xx
 // responses into this shape. Huma's default RFC 9457 problem+json would reach
-// them with an empty message, so every v2 error is rewritten into this shape by
-// tailscaleErrorTransformer. The HTTP status is read from the response code,
-// not from the body's status field.
+// them with an empty message, so tailscaleErrorTransformer rewrites every v2
+// error into this shape. The HTTP status is read from the response code, not
+// from the body's status field.
 type apiError struct {
 	Message string         `json:"message"`
 	Data    []apiErrorData `json:"data,omitempty"`

hscontrol/api/v2/keys.go

@@ -3,10 +3,13 @@ package apiv2
 import (
 	"context"
 	"net/http"
+	"strconv"
 	"time"
 
 	"github.com/danielgtaylor/huma/v2"
+	"github.com/juanfont/headscale/hscontrol/scope"
 	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/juanfont/headscale/hscontrol/util"
 )
 
 func init() {
@@ -17,9 +20,13 @@ func init() {
 // 90 days, matching Tailscale and the Terraform provider default.
 const defaultExpiry = 90 * 24 * time.Hour
 
-// keyTypeAuth is the only key kind Headscale issues; Tailscale's OAuth-client
-// and federated-identity kinds are out of scope.
-const keyTypeAuth = "auth"
+const (
+	// keyTypeAuth is a machine auth key (Headscale pre-auth key); the default.
+	keyTypeAuth = "auth"
+	// keyTypeClient is an OAuth client (client-credentials). Multiplexed onto the
+	// keys resource exactly as Tailscale does.
+	keyTypeClient = "client"
+)
 
 // KeyCapabilities maps a resource to the actions a key permits. Headscale
 // populates only devices.create (auth keys); the named types (vs Tailscale's
@@ -42,16 +49,27 @@ type KeyDeviceCreateCapabilities struct {
 	Tags []string `json:"tags"`
 }
 
-// CreateKeyRequest is the POST body. expirySeconds is plain seconds (unlike the
-// response, see Key).
+// CreateKeyRequest is the POST body. It is multiplexed by keyType: "auth"
+// (default) creates a machine auth key from Capabilities; "client" creates an
+// OAuth client from the top-level Scopes and Tags. expirySeconds is plain
+// seconds (unlike the response, see Key).
 type CreateKeyRequest struct {
-	Capabilities  KeyCapabilities `json:"capabilities"`
-	ExpirySeconds int64           `doc:"Lifetime in seconds; defaults to 90 days." json:"expirySeconds,omitempty"`
-	Description   string          `json:"description,omitempty"                    maxLength:"50"`
+	KeyType string `doc:"Key kind: \"auth\" (default) or \"client\" (OAuth client)." json:"keyType,omitempty"`
+	// Capabilities is optional: an auth key carries device-create capabilities,
+	// but an OAuth client (keyType:"client") has none and the Tailscale clients
+	// omit the field entirely, so it must not be required.
+	Capabilities  *KeyCapabilities `json:"capabilities,omitempty"`
+	ExpirySeconds int64            `doc:"Lifetime in seconds; defaults to 90 days. Auth keys only." json:"expirySeconds,omitempty"`
+	Description   string           `json:"description,omitempty"                                    maxLength:"50"`
+	// Scopes and Tags are top-level and apply only to keyType "client" (an OAuth
+	// client). Auth-key tags live under Capabilities.Devices.Create.Tags.
+	Scopes []string `doc:"OAuth scopes granted to the client. keyType=client only." json:"scopes,omitempty"`
+	Tags   []string `doc:"Tags the client may assign. keyType=client only."         json:"tags,omitempty"`
 }
 
-// Key is the Tailscale auth-key response. expirySeconds is emitted in seconds
-// to match the Tailscale spec; the secret key is present only at creation.
+// Key is the Tailscale key response, shared by auth keys and OAuth clients.
+// expirySeconds is emitted in seconds to match the Tailscale spec; the secret
+// key is present only at creation.
 type Key struct {
 	ID            string          `json:"id"`
 	KeyType       string          `json:"keyType"`
@@ -63,6 +81,7 @@ type Key struct {
 	Revoked       *time.Time      `json:"revoked,omitempty"`
 	Invalid       bool            `json:"invalid"`
 	Capabilities  KeyCapabilities `json:"capabilities"`
+	Scopes        []string        `json:"scopes,omitempty"`
 	Tags          []string        `json:"tags,omitempty"`
 	UserID        string          `json:"userId,omitempty"`
 }
@@ -98,14 +117,19 @@ type (
 	}
 )
 
+// The keys resource is multiplexed by keyType (auth key vs OAuth client), so the
+// scope an operation requires depends on the request rather than being fixed.
+// requireScope (which the middleware enforces statically) is therefore omitted
+// here; each handler authorizes via requireKeyScope once the kind is known.
 func registerKeys(api huma.API, b Backend) {
 	keysTags := []string{"Keys", "Tailscale compat"}
 
-	huma.Register(api, requireScope(huma.Operation{
+	huma.Register(api, huma.Operation{
 		OperationID: "createKey",
 		Method:      http.MethodPost,
 		Path:        "/api/v2/tailnet/{tailnet}/keys",
-		Summary:     "Create an auth key",
+		Summary:     "Create an auth key or OAuth client",
+		Description: "Requires the `auth_keys` scope for an auth key, or `oauth_keys` for an OAuth client (an admin API key is all-access).",
 		Tags:        keysTags,
 		Security:    security,
 		Errors: []int{
@@ -114,63 +138,25 @@ func registerKeys(api huma.API, b Backend) {
 			http.StatusForbidden,
 			http.StatusNotFound,
 		},
-	}, ScopeAuthKeys), func(ctx context.Context, in *createKeyInput) (*keyOutput, error) {
+	}, func(ctx context.Context, in *createKeyInput) (*keyOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
 		}
 
-		create := in.Body.Capabilities.Devices.Create
-
-		// Ownership: tags -> tagged key; no tags -> owned by the API key's user;
-		// no tags and an ownerless (legacy/admin) key -> 400.
-		var userID *types.UserID
-
-		if len(create.Tags) == 0 {
-			uid, ok := ownerUser(ctx)
-			if !ok {
-				return nil, huma.Error400BadRequest(
-					"an auth key without tags must be created with a user-owned API key",
-				)
-			}
-
-			userID = &uid
+		if in.Body.KeyType == keyTypeClient {
+			return createOAuthClient(ctx, b, in.Body)
 		}
 
-		expiration := time.Now().Add(expiryDuration(in.Body.ExpirySeconds))
-
-		pak, err := b.State.CreatePreAuthKey(
-			userID,
-			create.Reusable,
-			create.Ephemeral,
-			&expiration,
-			create.Tags,
-		)
-		if err != nil {
-			return nil, mapError("creating auth key", err)
-		}
-
-		if in.Body.Description != "" {
-			err := b.State.SetPreAuthKeyDescription(
-				pak.ID,
-				in.Body.Description,
-			)
-			if err != nil {
-				return nil, huma.Error500InternalServerError(
-					"setting auth key description",
-					err,
-				)
-			}
-		}
-
-		return &keyOutput{Body: keyFromNew(pak, in.Body)}, nil
+		return createAuthKey(ctx, b, in.Body)
 	})
 
-	huma.Register(api, requireScope(huma.Operation{
+	huma.Register(api, huma.Operation{
 		OperationID: "listKeys",
 		Method:      http.MethodGet,
 		Path:        "/api/v2/tailnet/{tailnet}/keys",
-		Summary:     "List auth keys",
+		Summary:     "List auth keys and OAuth clients",
+		Description: "A token sees the kinds it can read: `auth_keys:read` for auth keys, `oauth_keys:read` for OAuth clients (an admin API key sees all).",
 		Tags:        keysTags,
 		Security:    security,
 		Errors: []int{
@@ -178,32 +164,49 @@ func registerKeys(api huma.API, b Backend) {
 			http.StatusForbidden,
 			http.StatusNotFound,
 		},
-	}, ScopeAuthKeysRead), func(ctx context.Context, in *listKeysInput) (*listKeysOutput, error) {
+	}, func(ctx context.Context, in *listKeysInput) (*listKeysOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
 		}
 
-		keys, err := b.State.ListPreAuthKeys()
-		if err != nil {
-			return nil, huma.Error500InternalServerError("listing auth keys", err)
-		}
+		scopes, isOAuth := principalScopes(ctx)
 
 		out := &listKeysOutput{}
-		out.Body.Keys = make([]Key, 0, len(keys))
+		out.Body.Keys = []Key{}
 
-		for i := range keys {
-			out.Body.Keys = append(out.Body.Keys, keyFromStored(&keys[i]))
+		// A token sees the key kinds it has read scope for; an admin key sees all.
+		if !isOAuth || scope.Grants(scope.Parse(scopes), scope.AuthKeysRead) {
+			keys, err := b.State.ListPreAuthKeys()
+			if err != nil {
+				return nil, huma.Error500InternalServerError("listing auth keys", err)
+			}
+
+			for i := range keys {
+				out.Body.Keys = append(out.Body.Keys, keyFromStored(&keys[i]))
+			}
+		}
+
+		if !isOAuth || scope.Grants(scope.Parse(scopes), scope.OAuthKeysRead) {
+			clients, err := b.State.ListOAuthClients()
+			if err != nil {
+				return nil, huma.Error500InternalServerError("listing oauth clients", err)
+			}
+
+			for i := range clients {
+				out.Body.Keys = append(out.Body.Keys, oauthClientToKey(&clients[i], ""))
+			}
 		}
 
 		return out, nil
 	})
 
-	huma.Register(api, requireScope(huma.Operation{
+	huma.Register(api, huma.Operation{
 		OperationID: "getKey",
 		Method:      http.MethodGet,
 		Path:        "/api/v2/tailnet/{tailnet}/keys/{keyId}",
-		Summary:     "Get an auth key",
+		Summary:     "Get an auth key or OAuth client",
+		Description: "Requires `auth_keys:read` for an auth key, or `oauth_keys:read` for an OAuth client (an admin API key is all-access).",
 		Tags:        keysTags,
 		Security:    security,
 		Errors: []int{
@@ -211,12 +214,29 @@ func registerKeys(api huma.API, b Backend) {
 			http.StatusForbidden,
 			http.StatusNotFound,
 		},
-	}, ScopeAuthKeysRead), func(ctx context.Context, in *keyByIDInput) (*keyOutput, error) {
+	}, func(ctx context.Context, in *keyByIDInput) (*keyOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
 		}
 
+		// An OAuth client id is a hex string distinct from a numeric auth-key id,
+		// so a client lookup that hits is authoritative; otherwise fall through to
+		// the auth-key path. The lookup is gated on the caller actually holding
+		// oauth_keys:read so a token without it cannot tell a real client id (403)
+		// from an unknown key (404) — i.e. no client-existence oracle.
+		if requireKeyScope(ctx, scope.OAuthKeysRead) == nil {
+			client, err := b.State.GetOAuthClientByClientID(in.KeyID)
+			if err == nil {
+				return &keyOutput{Body: oauthClientToKey(client, "")}, nil
+			}
+		}
+
+		err = requireKeyScope(ctx, scope.AuthKeysRead)
+		if err != nil {
+			return nil, err
+		}
+
 		key, err := findKeyByID(b, in.KeyID)
 		if err != nil {
 			return nil, err
@@ -225,11 +245,12 @@ func registerKeys(api huma.API, b Backend) {
 		return &keyOutput{Body: keyFromStored(key)}, nil
 	})
 
-	huma.Register(api, requireScope(huma.Operation{
+	huma.Register(api, huma.Operation{
 		OperationID:   "deleteKey",
 		Method:        http.MethodDelete,
 		Path:          "/api/v2/tailnet/{tailnet}/keys/{keyId}",
-		Summary:       "Delete an auth key",
+		Summary:       "Delete an auth key or OAuth client",
+		Description:   "Requires the `auth_keys` scope for an auth key, or `oauth_keys` for an OAuth client (an admin API key is all-access).",
 		Tags:          keysTags,
 		Security:      security,
 		DefaultStatus: http.StatusOK,
@@ -238,12 +259,31 @@ func registerKeys(api huma.API, b Backend) {
 			http.StatusForbidden,
 			http.StatusNotFound,
 		},
-	}, ScopeAuthKeys), func(ctx context.Context, in *keyByIDInput) (*deleteKeyOutput, error) {
+	}, func(ctx context.Context, in *keyByIDInput) (*deleteKeyOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
 		}
 
+		// Gated on oauth_keys (write) for the same no-existence-oracle reason as
+		// getKey: a token without it must not learn that an id is an OAuth client.
+		if requireKeyScope(ctx, scope.OAuthKeys) == nil {
+			_, err = b.State.GetOAuthClientByClientID(in.KeyID)
+			if err == nil {
+				err = b.State.RevokeOAuthClient(in.KeyID)
+				if err != nil {
+					return nil, mapError("deleting oauth client", err)
+				}
+
+				return &deleteKeyOutput{}, nil
+			}
+		}
+
+		err = requireKeyScope(ctx, scope.AuthKeys)
+		if err != nil {
+			return nil, err
+		}
+
 		id, err := parseID(in.KeyID, "auth key")
 		if err != nil {
 			return nil, err
@@ -261,6 +301,163 @@ func registerKeys(api huma.API, b Backend) {
 	})
 }
 
+// requireKeyScope authorizes a keys operation for an OAuth access token. An admin
+// API key carries no OAuth scopes and is all-access, so it always passes.
+func requireKeyScope(ctx context.Context, need scope.Scope) error {
+	scopes, isOAuth := principalScopes(ctx)
+	if !isOAuth {
+		return nil
+	}
+
+	if !scope.Grants(scope.Parse(scopes), need) {
+		return huma.Error403Forbidden("token is missing the required scope " + string(need))
+	}
+
+	return nil
+}
+
+// createAuthKey creates a machine auth key (pre-auth key). Ownership: tags -> a
+// tagged key; no tags -> owned by the API key's user. An OAuth access token must
+// mint tagged keys, and each tag must be within the token's grant.
+func createAuthKey(ctx context.Context, b Backend, body CreateKeyRequest) (*keyOutput, error) {
+	err := requireKeyScope(ctx, scope.AuthKeys)
+	if err != nil {
+		return nil, err
+	}
+
+	var create KeyDeviceCreateCapabilities
+	if body.Capabilities != nil {
+		create = body.Capabilities.Devices.Create
+	}
+
+	tokenTags, isOAuth := principalTags(ctx)
+
+	var userID *types.UserID
+
+	switch {
+	case len(create.Tags) > 0:
+		// A key minted by an OAuth token may only carry tags within the token's
+		// grant (held directly, or owned by a held tag) and defined in policy,
+		// matching SetNodeTags. An admin key keeps the historical behaviour of
+		// validating only tag syntax (db.validateACLTags).
+		if isOAuth {
+			for _, tag := range create.Tags {
+				if !b.State.TagExists(tag) {
+					return nil, huma.Error400BadRequest("tag " + tag + " is not defined in policy")
+				}
+
+				if !b.State.TagOwnedByTags(tag, tokenTags) {
+					return nil, huma.Error403Forbidden(
+						"token may not assign tag " + tag,
+					)
+				}
+			}
+		}
+
+	case isOAuth:
+		// OAuth-minted keys are tailnet/tag-owned; an untagged (user-owned) key
+		// cannot be created from a token.
+		return nil, huma.Error403Forbidden("an OAuth client must create tagged auth keys")
+
+	default:
+		uid, ok := ownerUser(ctx)
+		if !ok {
+			return nil, huma.Error400BadRequest(
+				"an auth key without tags must be created with a user-owned API key",
+			)
+		}
+
+		userID = &uid
+	}
+
+	expiration := time.Now().Add(expiryDuration(body.ExpirySeconds))
+
+	pak, err := b.State.CreatePreAuthKey(
+		userID,
+		create.Reusable,
+		create.Ephemeral,
+		&expiration,
+		create.Tags,
+	)
+	if err != nil {
+		return nil, mapError("creating auth key", err)
+	}
+
+	if body.Description != "" {
+		err := b.State.SetPreAuthKeyDescription(pak.ID, body.Description)
+		if err != nil {
+			return nil, huma.Error500InternalServerError("setting auth key description", err)
+		}
+	}
+
+	return &keyOutput{Body: keyFromNew(pak, body)}, nil
+}
+
+// createOAuthClient creates an OAuth client (keyType:"client"). The client secret
+// is returned once, here.
+func createOAuthClient(ctx context.Context, b Backend, body CreateKeyRequest) (*keyOutput, error) {
+	err := requireKeyScope(ctx, scope.OAuthKeys)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(body.Scopes) == 0 {
+		return nil, huma.Error400BadRequest("an OAuth client must declare at least one scope")
+	}
+
+	// Tailscale: tags are mandatory when the scopes include devices:core or
+	// auth_keys, because such a client mints tagged, tailnet-owned credentials.
+	if scope.RequiresTags(scope.Parse(body.Scopes)) && len(body.Tags) == 0 {
+		return nil, huma.Error400BadRequest(
+			"tags are required when scopes include devices:core or auth_keys",
+		)
+	}
+
+	// A client created by an OAuth token may not be granted authority the token
+	// lacks: its scopes must each be within the token's grant, and its tags within
+	// the token's tags and defined in policy (matching SetNodeTags). Otherwise an
+	// oauth_keys token could mint an all-access client and escalate. An admin API
+	// key (not an OAuth token) is unrestricted and keeps the historical tag
+	// behaviour (syntax-only validation).
+	if tokenScopes, isOAuth := principalScopes(ctx); isOAuth {
+		for _, s := range body.Scopes {
+			if !scope.Grants(scope.Parse(tokenScopes), scope.Scope(s)) {
+				return nil, huma.Error403Forbidden(
+					"client may not be granted scope " + s + " beyond the creating token",
+				)
+			}
+		}
+
+		tokenTags, _ := principalTags(ctx)
+
+		for _, tag := range body.Tags {
+			if !b.State.TagExists(tag) {
+				return nil, huma.Error400BadRequest("tag " + tag + " is not defined in policy")
+			}
+
+			if !b.State.TagOwnedByTags(tag, tokenTags) {
+				return nil, huma.Error403Forbidden(
+					"client may not be granted tag " + tag + " beyond the creating token",
+				)
+			}
+		}
+	}
+
+	var creator *uint
+
+	if uid, ok := ownerUser(ctx); ok {
+		u := uint(uid)
+		creator = &u
+	}
+
+	secret, client, err := b.State.CreateOAuthClient(body.Scopes, body.Tags, body.Description, creator)
+	if err != nil {
+		return nil, mapError("creating oauth client", err)
+	}
+
+	return &keyOutput{Body: oauthClientToKey(client, secret)}, nil
+}
+
 // findKeyByID looks up a stored pre-auth key by its (stringified) id with a
 // direct by-id query; an unknown id surfaces as gorm.ErrRecordNotFound, which
 // mapError turns into a 404.
@@ -283,7 +480,10 @@ func findKeyByID(b Backend, rawID string) (*types.PreAuthKey, error) {
 // match keyFromStored: Headscale always authorizes pre-auth-key nodes, so the
 // create and read paths must agree or the Terraform provider sees a diff.
 func keyFromNew(pak *types.PreAuthKeyNew, req CreateKeyRequest) Key {
-	create := req.Capabilities.Devices.Create
+	var create KeyDeviceCreateCapabilities
+	if req.Capabilities != nil {
+		create = req.Capabilities.Devices.Create
+	}
 
 	key := Key{
 		ID:          pak.StringID(),
@@ -344,6 +544,32 @@ func keyFromStored(pak *types.PreAuthKey) Key {
 	return key
 }
 
+// oauthClientToKey builds the keys response for an OAuth client. secret is the
+// plaintext client secret, set only on the create response and empty on get/list
+// (the secret is never re-exposed).
+func oauthClientToKey(client *types.OAuthClient, secret string) Key {
+	key := Key{
+		ID:          client.ClientID,
+		KeyType:     keyTypeClient,
+		Key:         secret,
+		Description: client.Description,
+		Created:     timeOrZero(client.CreatedAt),
+		Scopes:      emptyIfNil(client.Scopes),
+		Tags:        emptyIfNil(client.Tags),
+	}
+
+	if client.Revoked != nil {
+		key.Revoked = client.Revoked
+		key.Invalid = true
+	}
+
+	if client.UserID != nil {
+		key.UserID = strconv.FormatUint(uint64(*client.UserID), util.Base10)
+	}
+
+	return key
+}
+
 func capabilities(
 	reusable, ephemeral, preauthorized bool,
 	tags []string,

hscontrol/api/v2/oauth.go

@@ -0,0 +1,178 @@
+package apiv2
+
+import (
+	"encoding/json"
+	"net/http"
+	"slices"
+	"strings"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/juanfont/headscale/hscontrol/scope"
+	"github.com/juanfont/headscale/hscontrol/types"
+)
+
+// accessTokenTTL is the fixed lifetime of a minted access token, matching
+// Tailscale's non-configurable one hour.
+const accessTokenTTL = time.Hour
+
+// registerOAuthToken mounts POST /api/v2/oauth/token on the router. It is a plain
+// handler, not a Huma operation: it consumes application/x-www-form-urlencoded
+// and emits RFC 6749 OAuth2 error bodies ({"error","error_description"}), neither
+// of which fits Huma's JSON-in / Tailscale-error-out machinery.
+// ponytail: one bespoke OAuth endpoint isn't worth bending Huma around.
+func registerOAuthToken(router chi.Router, b Backend) {
+	router.Post("/api/v2/oauth/token", oauthTokenHandler(b))
+}
+
+// tokenResponse is the OAuth 2.0 client-credentials success body.
+type tokenResponse struct {
+	AccessToken string `json:"access_token"`
+	TokenType   string `json:"token_type"`
+	ExpiresIn   int    `json:"expires_in"`
+	Scope       string `json:"scope,omitempty"`
+}
+
+// oauthTokenHandler implements the client-credentials grant (RFC 6749 §4.4):
+// authenticate the client, optionally narrow the granted scopes/tags, and mint a
+// short-lived bearer token.
+func oauthTokenHandler(b Backend) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		err := r.ParseForm()
+		if err != nil {
+			writeOAuthError(w, http.StatusBadRequest, "invalid_request", "could not parse request body")
+
+			return
+		}
+
+		// grant_type defaults to client_credentials: Tailscale's documented curl
+		// omits it, and the x/oauth2 client always sends it.
+		if gt := r.PostForm.Get("grant_type"); gt != "" && gt != "client_credentials" {
+			writeOAuthError(w, http.StatusBadRequest, "unsupported_grant_type",
+				"only the client_credentials grant is supported")
+
+			return
+		}
+
+		// Credentials may arrive in the body or as HTTP Basic (the x/oauth2
+		// auto-detect probes Basic first). The secret embeds the client id, so a
+		// separate client_id is not required.
+		secret := r.PostForm.Get("client_secret")
+		if secret == "" {
+			if _, pass, ok := r.BasicAuth(); ok {
+				secret = pass
+			}
+		}
+
+		if secret == "" {
+			writeOAuthError(w, http.StatusUnauthorized, "invalid_client", "missing client credentials")
+
+			return
+		}
+
+		client, err := b.State.AuthenticateOAuthClient(secret)
+		if err != nil {
+			writeOAuthError(w, http.StatusUnauthorized, "invalid_client", "invalid client credentials")
+
+			return
+		}
+
+		// Optional space-delimited scope/tags narrow the token to a subset of the
+		// client's grant.
+		scopes, badScope, ok := narrowScopes(client.Scopes, strings.Fields(r.PostForm.Get("scope")))
+		if !ok {
+			writeOAuthError(w, http.StatusBadRequest, "invalid_scope",
+				"scope "+badScope+" is not granted to this client")
+
+			return
+		}
+
+		tags, badTag, ok := narrowTags(client, strings.Fields(r.PostForm.Get("tags")))
+		if !ok {
+			writeOAuthError(w, http.StatusBadRequest, "invalid_target",
+				"tag "+badTag+" is not granted to this client")
+
+			return
+		}
+
+		expiry := time.Now().Add(accessTokenTTL)
+
+		tokenStr, _, err := b.State.MintAccessToken(client.ClientID, scopes, tags, &expiry)
+		if err != nil {
+			writeOAuthError(w, http.StatusInternalServerError, "server_error", "could not mint access token")
+
+			return
+		}
+
+		writeJSON(w, http.StatusOK, tokenResponse{
+			AccessToken: tokenStr,
+			TokenType:   "Bearer",
+			ExpiresIn:   int(accessTokenTTL.Seconds()),
+			Scope:       strings.Join(scopes, " "),
+		})
+	}
+}
+
+// narrowScopes returns the requested scopes if each is granted by the client (an
+// empty request means "the client's full grant"), otherwise the offending scope
+// and false. A client holding a broad scope (e.g. "all") may mint a token limited
+// to a narrower one.
+func narrowScopes(granted, requested []string) ([]string, string, bool) {
+	if len(requested) == 0 {
+		return granted, "", true
+	}
+
+	for _, req := range requested {
+		if !scope.Grants(scope.Parse(granted), scope.Scope(req)) {
+			return nil, req, false
+		}
+	}
+
+	return requested, "", true
+}
+
+// narrowTags returns the requested tags if each is within the client's grant (an
+// empty request means "the client's full tag set"), otherwise the offending tag
+// and false. A client with the "all" scope may assign any tag, matching Tailscale.
+func narrowTags(client *types.OAuthClient, requested []string) ([]string, string, bool) {
+	if len(requested) == 0 {
+		return client.Tags, "", true
+	}
+
+	allScope := slices.Contains(client.Scopes, string(scope.All))
+
+	for _, req := range requested {
+		// Reject malformed tags at the trust boundary. A client's own tags are
+		// validated at creation, so this only matters for an "all"-scope client,
+		// whose tags would otherwise skip the membership check below and flow
+		// unvalidated into auth-key creation.
+		if !strings.HasPrefix(req, "tag:") {
+			return nil, req, false
+		}
+
+		if !allScope && !slices.Contains(client.Tags, req) {
+			return nil, req, false
+		}
+	}
+
+	return requested, "", true
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	// Token responses carry bearer credentials; RFC 6749 §5.1 forbids caching
+	// them. writeJSON serves only the token endpoint, so set it unconditionally.
+	w.Header().Set("Cache-Control", "no-store")
+	w.Header().Set("Pragma", "no-cache")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v) //nolint:errchkjson // best-effort response write of a known-safe value
+}
+
+// writeOAuthError emits an RFC 6749 §5.2 error body, which the x/oauth2 client
+// parses into its RetrieveError.
+func writeOAuthError(w http.ResponseWriter, status int, code, desc string) {
+	writeJSON(w, status, map[string]string{
+		"error":             code,
+		"error_description": desc,
+	})
+}

hscontrol/api/v2/oauth_test.go

@@ -0,0 +1,31 @@
+package apiv2
+
+import (
+	"testing"
+
+	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestNarrowTagsRejectsMalformed asserts the token endpoint validates tag
+// format at the trust boundary. An "all"-scope client may assign any tag, but a
+// malformed tag (missing the "tag:" prefix) must still be rejected rather than
+// flowing unvalidated into auth-key creation.
+func TestNarrowTagsRejectsMalformed(t *testing.T) {
+	all := &types.OAuthClient{Scopes: []string{"all"}}
+
+	_, bad, ok := narrowTags(all, []string{"not-a-tag"})
+	assert.False(t, ok, "malformed tag must be rejected even with all scope")
+	assert.Equal(t, "not-a-tag", bad)
+
+	got, _, ok := narrowTags(all, []string{"tag:anything"})
+	assert.True(t, ok, "all scope may assign any well-formed tag")
+	assert.Equal(t, []string{"tag:anything"}, got)
+
+	// A scoped client may only assign its own well-formed tags.
+	scoped := &types.OAuthClient{Scopes: []string{"auth_keys"}, Tags: []string{"tag:ci"}}
+
+	_, bad, ok = narrowTags(scoped, []string{"tag:other"})
+	assert.False(t, ok)
+	assert.Equal(t, "tag:other", bad)
+}

hscontrol/api/v2/settings.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/danielgtaylor/huma/v2"
+	"github.com/juanfont/headscale/hscontrol/scope"
 	"github.com/juanfont/headscale/hscontrol/types"
 )
 
@@ -59,7 +60,7 @@ func registerSettings(api huma.API, b Backend) {
 		Tags:        settingsTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeFeatureSettingsRead), func(ctx context.Context, in *getSettingsInput) (*settingsOutput, error) {
+	}, scope.FeatureSettingsRead), func(ctx context.Context, in *getSettingsInput) (*settingsOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err
@@ -86,7 +87,7 @@ func registerSettings(api huma.API, b Backend) {
 		// The body is accepted but ignored; skip validation.
 		SkipValidateBody: true,
 		Errors:           []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound, http.StatusNotImplemented},
-	}, ScopeFeatureSettings), func(ctx context.Context, in *patchSettingsInput) (*settingsOutput, error) {
+	}, scope.FeatureSettings), func(ctx context.Context, in *patchSettingsInput) (*settingsOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err

hscontrol/api/v2/users.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/danielgtaylor/huma/v2"
+	"github.com/juanfont/headscale/hscontrol/scope"
 	"github.com/juanfont/headscale/hscontrol/types"
 )
 
@@ -73,7 +74,7 @@ func registerUsers(api huma.API, b Backend) {
 		Tags:        usersTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeUsersRead), func(ctx context.Context, in *userByIDInput) (*userOutput, error) {
+	}, scope.UsersRead), func(ctx context.Context, in *userByIDInput) (*userOutput, error) {
 		view, err := lookupUser(b, in.UserID)
 		if err != nil {
 			return nil, err
@@ -90,7 +91,7 @@ func registerUsers(api huma.API, b Backend) {
 		Tags:        usersTags,
 		Security:    security,
 		Errors:      []int{http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound},
-	}, ScopeUsersRead), func(ctx context.Context, in *listUsersInput) (*listUsersOutput, error) {
+	}, scope.UsersRead), func(ctx context.Context, in *listUsersInput) (*listUsersOutput, error) {
 		err := requireDefaultTailnet(in.Tailnet)
 		if err != nil {
 			return nil, err

hscontrol/apiv2_keys_test.go

@@ -17,8 +17,8 @@ import (
 )
 
 // taggedCaps builds capabilities for a tagged auth key.
-func taggedCaps(tags ...string) apiv2.KeyCapabilities {
-	return apiv2.KeyCapabilities{
+func taggedCaps(tags ...string) *apiv2.KeyCapabilities {
+	return &apiv2.KeyCapabilities{
 		Devices: apiv2.KeyDeviceCapabilities{Create: apiv2.KeyDeviceCreateCapabilities{
 			Reusable:      true,
 			Preauthorized: true,
@@ -138,7 +138,7 @@ func TestAPIv2Key_Create_Tagged(t *testing.T) {
 		KeyType:       "auth",
 		Description:   "dev access",
 		ExpirySeconds: 86400,
-		Capabilities:  taggedCaps("tag:test"),
+		Capabilities:  *taggedCaps("tag:test"),
 		Tags:          []string{"tag:test"},
 	}
 	if diff := cmp.Diff(want, created, cmpopts.IgnoreFields(apiv2.Key{}, "ID", "Key", "Created", "Expires")); diff != "" {
@@ -172,7 +172,7 @@ func TestAPIv2Key_Create_Permutations(t *testing.T) {
 	}{
 		{
 			name: "single-use",
-			req: apiv2.CreateKeyRequest{Capabilities: apiv2.KeyCapabilities{
+			req: apiv2.CreateKeyRequest{Capabilities: &apiv2.KeyCapabilities{
 				Devices: apiv2.KeyDeviceCapabilities{Create: apiv2.KeyDeviceCreateCapabilities{Tags: []string{"tag:test"}}},
 			}},
 			wantSeconds: 7776000,
@@ -185,7 +185,7 @@ func TestAPIv2Key_Create_Permutations(t *testing.T) {
 		},
 		{
 			name: "ephemeral",
-			req: apiv2.CreateKeyRequest{Capabilities: apiv2.KeyCapabilities{
+			req: apiv2.CreateKeyRequest{Capabilities: &apiv2.KeyCapabilities{
 				Devices: apiv2.KeyDeviceCapabilities{Create: apiv2.KeyDeviceCreateCapabilities{
 					Ephemeral: true,
 					Tags:      []string{"tag:test"},
@@ -246,7 +246,7 @@ func TestAPIv2Key_Get(t *testing.T) {
 		KeyType:       "auth",
 		Description:   "dev access",
 		ExpirySeconds: 86400,
-		Capabilities:  taggedCaps("tag:test"),
+		Capabilities:  *taggedCaps("tag:test"),
 		Tags:          []string{"tag:test"},
 	}
 	if diff := cmp.Diff(want, got, cmpopts.IgnoreFields(apiv2.Key{}, "Created", "Expires")); diff != "" {
@@ -313,7 +313,7 @@ func TestAPIv2Key_Create_NoTags_NoOwner_400(t *testing.T) {
 
 	before := keyCount(t, app)
 
-	resp := api.Post("/api/v2/tailnet/-/keys", apiv2.CreateKeyRequest{Capabilities: apiv2.KeyCapabilities{}})
+	resp := api.Post("/api/v2/tailnet/-/keys", apiv2.CreateKeyRequest{Capabilities: &apiv2.KeyCapabilities{}})
 	assert.Equal(t, http.StatusBadRequest, resp.Code)
 	assert.Contains(t, resp.Body.String(), `"message"`)
 

hscontrol/scope/scope.go

@@ -40,6 +40,9 @@ const (
 
 	FeatureSettings     Scope = "feature_settings"
 	FeatureSettingsRead Scope = "feature_settings:read"
+
+	Users     Scope = "users"
+	UsersRead Scope = "users:read"
 )
 
 const readSuffix = ":read"
@@ -55,6 +58,7 @@ func Known() []Scope {
 		DevicesRoutes, DevicesRoutesRead,
 		PolicyFile, PolicyFileRead,
 		FeatureSettings, FeatureSettingsRead,
+		Users, UsersRead,
 	}
 }
 
@@ -69,7 +73,7 @@ func (s Scope) IsWrite() bool {
 }
 
 // Parse converts scope strings (as stored on a token or client) into Scope values.
-// Unknown strings are kept verbatim; they simply never satisfy any required scope.
+// Unknown strings are kept as-is; they simply never satisfy any required scope.
 func Parse(ss []string) []Scope {
 	out := make([]Scope, len(ss))
 	for i, s := range ss {