cmd/derper: fix mesh auth for DERP servers (#16061)

To authenticate mesh keys, the DERP servers used a simple == comparison,
which is susceptible to a side channel timing attack.

By extracting the mesh key for a DERP server, an attacker could DoS it
by forcing disconnects using derp.Client.ClosePeer. They could also
enumerate the public Wireguard keys, IP addresses and ports for nodes
connected to that DERP server.

DERP servers configured without mesh keys deny all such requests.

This patch also extracts the mesh key logic into key.DERPMesh, to
prevent this from happening again.

Security bulletin: https://tailscale.com/security-bulletins#ts-2025-003

Fixes tailscale/corp#28720

Signed-off-by: Simon Law <sfllaw@tailscale.com>

derp/derp_client.go

@@ -30,7 +30,7 @@ type Client struct {
 	logf        logger.Logf
 	nc          Conn
 	br          *bufio.Reader
-	meshKey     string
+	meshKey     key.DERPMesh
 	canAckPings bool
 	isProber    bool
 
@@ -56,7 +56,7 @@ func (f clientOptFunc) update(o *clientOpt) { f(o) }
 
 // clientOpt are the options passed to newClient.
 type clientOpt struct {
-	MeshKey     string
+	MeshKey     key.DERPMesh
 	ServerPub   key.NodePublic
 	CanAckPings bool
 	IsProber    bool
@@ -66,7 +66,7 @@ type clientOpt struct {
 // access to join the mesh.
 //
 // An empty key means to not use a mesh key.
-func MeshKey(key string) ClientOpt { return clientOptFunc(func(o *clientOpt) { o.MeshKey = key }) }
+func MeshKey(k key.DERPMesh) ClientOpt { return clientOptFunc(func(o *clientOpt) { o.MeshKey = k }) }
 
 // IsProber returns a ClientOpt to pass to the DERP server during connect to
 // declare that this client is a a prober.
@@ -182,7 +182,7 @@ type clientInfo struct {
 func (c *Client) sendClientKey() error {
 	msg, err := json.Marshal(clientInfo{
 		Version:     ProtocolVersion,
-		MeshKey:     c.meshKey,
+		MeshKey:     c.meshKey.String(),
 		CanAckPings: c.canAckPings,
 		IsProber:    c.isProber,
 	})

derp/derp_server.go

@@ -134,7 +134,7 @@ type Server struct {
 	publicKey   key.NodePublic
 	logf        logger.Logf
 	memSys0     uint64 // runtime.MemStats.Sys at start (or early-ish)
-	meshKey     string
+	meshKey     key.DERPMesh
 	limitedLogf logger.Logf
 	metaCert    []byte // the encoded x509 cert to send after LetsEncrypt cert+intermediate
 	dupPolicy   dupPolicy
@@ -464,8 +464,13 @@ func genDroppedCounters() {
 // amongst themselves.
 //
 // It must be called before serving begins.
-func (s *Server) SetMeshKey(v string) {
-	s.meshKey = v
+func (s *Server) SetMeshKey(v string) error {
+	k, err := key.ParseDERPMesh(v)
+	if err != nil {
+		return err
+	}
+	s.meshKey = k
+	return nil
 }
 
 // SetVerifyClients sets whether this DERP server verifies clients through tailscaled.
@@ -506,10 +511,10 @@ func (s *Server) SetTCPWriteTimeout(d time.Duration) {
 }
 
 // HasMeshKey reports whether the server is configured with a mesh key.
-func (s *Server) HasMeshKey() bool { return s.meshKey != "" }
+func (s *Server) HasMeshKey() bool { return !s.meshKey.IsZero() }
 
 // MeshKey returns the configured mesh key, if any.
-func (s *Server) MeshKey() string { return s.meshKey }
+func (s *Server) MeshKey() key.DERPMesh { return s.meshKey }
 
 // PrivateKey returns the server's private key.
 func (s *Server) PrivateKey() key.NodePrivate { return s.privateKey }
@@ -1355,7 +1360,18 @@ func (c *sclient) requestMeshUpdate() {
 // isMeshPeer reports whether the client is a trusted mesh peer
 // node in the DERP region.
 func (s *Server) isMeshPeer(info *clientInfo) bool {
-	return info != nil && info.MeshKey != "" && info.MeshKey == s.meshKey
+	// Compare mesh keys in constant time to prevent timing attacks.
+	// Since mesh keys are a fixed length, we don’t need to be concerned
+	// about timing attacks on client mesh keys that are the wrong length.
+	// See https://github.com/tailscale/corp/issues/28720
+	if info == nil || info.MeshKey == "" {
+		return false
+	}
+	k, err := key.ParseDERPMesh(info.MeshKey)
+	if err != nil {
+		return false
+	}
+	return s.meshKey.Equal(k)
 }
 
 // verifyClient checks whether the client is allowed to connect to the derper,

derp/derp_test.go

@@ -511,11 +511,13 @@ func (ts *testServer) close(t *testing.T) error {
 	return nil
 }
 
+const testMeshKey = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+
 func newTestServer(t *testing.T, ctx context.Context) *testServer {
 	t.Helper()
 	logf := logger.WithPrefix(t.Logf, "derp-server: ")
 	s := NewServer(key.NewNode(), logf)
-	s.SetMeshKey("mesh-key")
+	s.SetMeshKey(testMeshKey)
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
@@ -591,8 +593,12 @@ func newRegularClient(t *testing.T, ts *testServer, name string) *testClient {
 
 func newTestWatcher(t *testing.T, ts *testServer, name string) *testClient {
 	return newTestClient(t, ts, name, func(nc net.Conn, priv key.NodePrivate, logf logger.Logf) (*Client, error) {
+		mk, err := key.ParseDERPMesh(testMeshKey)
+		if err != nil {
+			return nil, err
+		}
 		brw := bufio.NewReadWriter(bufio.NewReader(nc), bufio.NewWriter(nc))
-		c, err := NewClient(priv, nc, brw, logf, MeshKey("mesh-key"))
+		c, err := NewClient(priv, nc, brw, logf, MeshKey(mk))
 		if err != nil {
 			return nil, err
 		}
@@ -1627,3 +1633,96 @@ func TestGetPerClientSendQueueDepth(t *testing.T) {
 		})
 	}
 }
+
+func TestSetMeshKey(t *testing.T) {
+	for name, tt := range map[string]struct {
+		key     string
+		want    key.DERPMesh
+		wantErr bool
+	}{
+		"clobber": {
+			key:     testMeshKey,
+			wantErr: false,
+		},
+		"invalid": {
+			key:     "badf00d",
+			wantErr: true,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			s := &Server{}
+
+			err := s.SetMeshKey(tt.key)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected err")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected err: %v", err)
+			}
+
+			want, err := key.ParseDERPMesh(tt.key)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !s.meshKey.Equal(want) {
+				t.Fatalf("got %v, want %v", s.meshKey, want)
+			}
+		})
+	}
+}
+
+func TestIsMeshPeer(t *testing.T) {
+	s := &Server{}
+	err := s.SetMeshKey(testMeshKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for name, tt := range map[string]struct {
+		info       *clientInfo
+		want       bool
+		wantAllocs float64
+	}{
+		"nil": {
+			info:       nil,
+			want:       false,
+			wantAllocs: 0,
+		},
+		"empty": {
+			info:       &clientInfo{MeshKey: ""},
+			want:       false,
+			wantAllocs: 0,
+		},
+		"invalid": {
+			info:       &clientInfo{MeshKey: "invalid"},
+			want:       false,
+			wantAllocs: 2, // error message
+		},
+		"mismatch": {
+			info:       &clientInfo{MeshKey: "0badf00d00000000000000000000000000000000000000000000000000000000"},
+			want:       false,
+			wantAllocs: 1,
+		},
+		"match": {
+			info:       &clientInfo{MeshKey: testMeshKey},
+			want:       true,
+			wantAllocs: 1,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			var got bool
+			allocs := testing.AllocsPerRun(1, func() {
+				got = s.isMeshPeer(tt.info)
+			})
+			if got != tt.want {
+				t.Fatalf("got %t, want %t: info = %#v", got, tt.want, tt.info)
+			}
+
+			if allocs != tt.wantAllocs && tt.want {
+				t.Errorf("%f allocations, want %f", allocs, tt.wantAllocs)
+			}
+		})
+	}
+}

derp/derphttp/derphttp_client.go

@@ -57,7 +57,7 @@ type Client struct {
 	TLSConfig     *tls.Config        // optional; nil means default
 	HealthTracker *health.Tracker    // optional; used if non-nil only
 	DNSCache      *dnscache.Resolver // optional; nil means no caching
-	MeshKey       string             // optional; for trusted clients
+	MeshKey       key.DERPMesh       // optional; for trusted clients
 	IsProber      bool               // optional; for probers to optional declare themselves as such
 
 	// WatchConnectionChanges is whether the client wishes to subscribe to

derp/derphttp/derphttp_test.go

@@ -212,6 +212,8 @@ func TestPing(t *testing.T) {
 	}
 }
 
+const testMeshKey = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+
 func newTestServer(t *testing.T, k key.NodePrivate) (serverURL string, s *derp.Server) {
 	s = derp.NewServer(k, t.Logf)
 	httpsrv := &http.Server{
@@ -224,7 +226,7 @@ func newTestServer(t *testing.T, k key.NodePrivate) (serverURL string, s *derp.S
 		t.Fatal(err)
 	}
 	serverURL = "http://" + ln.Addr().String()
-	s.SetMeshKey("1234")
+	s.SetMeshKey(testMeshKey)
 
 	go func() {
 		if err := httpsrv.Serve(ln); err != nil {
@@ -243,7 +245,11 @@ func newWatcherClient(t *testing.T, watcherPrivateKey key.NodePrivate, serverToW
 	if err != nil {
 		t.Fatal(err)
 	}
-	c.MeshKey = "1234"
+	k, err := key.ParseDERPMesh(testMeshKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c.MeshKey = k
 	return
 }