mapper: filter peer-change patches by ACL visibility

buildFromChange added PeersChangedPatch (online/offline, endpoint, key-expiry) directly, skipping the policy.ReduceNodes visibility filter that buildTailPeers applies to full peers. A node thus received the existence, presence, and addresses of peers its ACL forbids accessing. Restrict patches to the recipient's visible peer set.

hscontrol/mapper/mapper.go

@@ -428,8 +428,9 @@ func (m *mapper) buildFromChange(
 		}
 	}
 
-	if len(resp.PeerPatches) > 0 {
+	patches := m.filterVisiblePeerPatches(nodeID, resp.PeerPatches)
-		builder.WithPeerChangedPatch(resp.PeerPatches)
+	if len(patches) > 0 {
+		builder.WithPeerChangedPatch(patches)
 	}
 
 	if resp.PingRequest != nil {
@@ -439,6 +440,49 @@ func (m *mapper) buildFromChange(
 	return builder.Build()
 }
 
+// filterVisiblePeerPatches drops peer-change patches whose target peer the
+// recipient cannot see under the ACL policy, mirroring the [policy.ReduceNodes]
+// visibility filter [MapResponseBuilder.buildTailPeers] applies to full peer
+// objects. Without it, online/offline, endpoint, and key-expiry patches
+// disclose the existence, presence, and addresses of peers the recipient's
+// policy forbids it from accessing.
+func (m *mapper) filterVisiblePeerPatches(
+	nodeID types.NodeID,
+	patches []*tailcfg.PeerChange,
+) []*tailcfg.PeerChange {
+	node, ok := m.state.GetNodeByID(nodeID)
+	if !ok {
+		return nil
+	}
+
+	matchers, err := m.state.MatchersForNode(node)
+	if err != nil {
+		// Fail closed: if visibility cannot be resolved, send no patches
+		// rather than risk leaking peers the node may not access.
+		return nil
+	}
+
+	// No matchers means no policy restrictions, so every peer is visible —
+	// the same default buildTailPeers applies.
+	if len(matchers) == 0 {
+		return patches
+	}
+
+	var filtered []*tailcfg.PeerChange
+	for _, patch := range patches {
+		peer, ok := m.state.GetNodeByID(types.NodeID(patch.NodeID))
+		if !ok {
+			continue
+		}
+
+		if node.CanAccess(matchers, peer) || peer.CanAccess(matchers, node) {
+			filtered = append(filtered, patch)
+		}
+	}
+
+	return filtered
+}
+
 func writeDebugMapResponse(
 	resp *tailcfg.MapResponse,
 	t debugType,

hscontrol/mapper/mapper_test.go

@@ -7,7 +7,12 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
+	"github.com/juanfont/headscale/hscontrol/db"
+	"github.com/juanfont/headscale/hscontrol/state"
 	"github.com/juanfont/headscale/hscontrol/types"
+	"github.com/juanfont/headscale/hscontrol/types/change"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/dnstype"
 )
@@ -193,3 +198,86 @@ func TestNextDNSCapMapRendering(t *testing.T) {
 		}
 	})
 }
+
+// TestBuildFromChangeFiltersPeerPatchesByVisibility proves that incremental
+// peer-change patches (online/offline, endpoint, key-expiry) are restricted to
+// the recipient's ACL-visible peer set, the same way buildTailPeers filters
+// full peer objects via policy.ReduceNodes. Without it, a node receives the
+// existence, presence, and addresses of peers its policy forbids accessing.
+func TestBuildFromChangeFiltersPeerPatchesByVisibility(t *testing.T) {
+	tmp := t.TempDir()
+
+	p4 := netip.MustParsePrefix("100.64.0.0/10")
+	p6 := netip.MustParsePrefix("fd7a:115c:a1e0::/48")
+
+	cfg := &types.Config{
+		Database: types.DatabaseConfig{
+			Type:   types.DatabaseSqlite,
+			Sqlite: types.SqliteConfig{Path: tmp + "/h.db"},
+		},
+		PrefixV4:     &p4,
+		PrefixV6:     &p6,
+		IPAllocation: types.IPAllocationStrategySequential,
+		BaseDomain:   "headscale.test",
+		Policy:       types.PolicyConfig{Mode: types.PolicyModeDB},
+		DERP: types.DERPConfig{
+			DERPMap: &tailcfg.DERPMap{
+				Regions: map[int]*tailcfg.DERPRegion{999: {RegionID: 999}},
+			},
+		},
+		Tuning: types.Tuning{
+			NodeStoreBatchSize:    state.TestBatchSize,
+			NodeStoreBatchTimeout: state.TestBatchTimeout,
+		},
+	}
+
+	database, err := db.NewHeadscaleDatabase(cfg)
+	require.NoError(t, err)
+
+	user1 := database.CreateUserForTest("u1")
+	user2 := database.CreateUserForTest("u2")
+	n1 := database.CreateRegisteredNodeForTest(user1, "n1")
+	n1b := database.CreateRegisteredNodeForTest(user1, "n1b")
+	n2 := database.CreateRegisteredNodeForTest(user2, "n2")
+	require.NoError(t, database.Close())
+
+	s, err := state.NewState(cfg)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = s.Close() })
+
+	// Each user may reach only its own devices, so n1 cannot access n2.
+	policy := `{"acls":[
+		{"action":"accept","src":["u1@"],"dst":["u1@:*"]},
+		{"action":"accept","src":["u2@"],"dst":["u2@:*"]}
+	]}`
+	_, err = s.SetPolicy([]byte(policy))
+	require.NoError(t, err)
+
+	m := &mapper{state: s, cfg: cfg}
+
+	// n2 (user2) comes online; n1 (user1) must NOT receive its patch.
+	leakChange := change.NodeOnline(n2.ID)
+	resp, err := m.buildFromChange(n1.ID, tailcfg.CurrentCapabilityVersion, &leakChange)
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+
+	for _, p := range resp.PeersChangedPatch {
+		assert.NotEqual(t, n2.ID.NodeID(), p.NodeID,
+			"n1 must not receive an online patch for n2, which its policy forbids accessing")
+	}
+
+	// Control: n1b (same user) coming online IS visible to n1.
+	okChange := change.NodeOnline(n1b.ID)
+	resp2, err := m.buildFromChange(n1.ID, tailcfg.CurrentCapabilityVersion, &okChange)
+	require.NoError(t, err)
+	require.NotNil(t, resp2)
+
+	var gotVisible bool
+	for _, p := range resp2.PeersChangedPatch {
+		if p.NodeID == n1b.ID.NodeID() {
+			gotVisible = true
+		}
+	}
+	assert.True(t, gotVisible,
+		"n1 must receive the online patch for visible same-user peer n1b")
+}