[management, client] Fix SSH server audience validator (#5105)

* **New Features**
  * SSH server JWT validation now accepts multiple audiences with backward-compatible handling of the previous single-audience setting and a guard ensuring at least one audience is configured.
* **Tests**
  * Test suites updated and new tests added to cover multiple-audience scenarios and compatibility with existing behavior.
* **Other**
  * Startup logging enhanced to report configured audiences for JWT auth.

client/ssh/server/jwt_test.go

@@ -43,7 +43,7 @@ func TestJWTEnforcement(t *testing.T) {
 	t.Run("blocks_without_jwt", func(t *testing.T) {
 		jwtConfig := &JWTConfig{
 			Issuer:       "test-issuer",
-			Audience:     "test-audience",
+			Audiences:    []string{"test-audience"},
 			KeysLocation: "test-keys",
 		}
 		serverConfig := &Config{
@@ -202,7 +202,7 @@ func TestJWTDetection(t *testing.T) {
 
 	jwtConfig := &JWTConfig{
 		Issuer:       issuer,
-		Audience:     audience,
+		Audiences:    []string{audience},
 		KeysLocation: jwksURL,
 	}
 	serverConfig := &Config{
@@ -329,7 +329,7 @@ func TestJWTFailClose(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			jwtConfig := &JWTConfig{
 				Issuer:       issuer,
-				Audience:     audience,
+				Audiences:    []string{audience},
 				KeysLocation: jwksURL,
 				MaxTokenAge:  3600,
 			}
@@ -567,7 +567,7 @@ func TestJWTAuthentication(t *testing.T) {
 
 			jwtConfig := &JWTConfig{
 				Issuer:       issuer,
-				Audience:     audience,
+				Audiences:    []string{audience},
 				KeysLocation: jwksURL,
 			}
 			serverConfig := &Config{
@@ -646,3 +646,108 @@ func TestJWTAuthentication(t *testing.T) {
 		})
 	}
 }
+
+// TestJWTMultipleAudiences tests JWT validation with multiple audiences (dashboard and CLI).
+func TestJWTMultipleAudiences(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping JWT multiple audiences tests in short mode")
+	}
+
+	jwksServer, privateKey, jwksURL := setupJWKSServer(t)
+	defer jwksServer.Close()
+
+	const (
+		issuer            = "https://test-issuer.example.com"
+		dashboardAudience = "dashboard-audience"
+		cliAudience       = "cli-audience"
+	)
+
+	hostKey, err := nbssh.GeneratePrivateKey(nbssh.ED25519)
+	require.NoError(t, err)
+
+	testCases := []struct {
+		name       string
+		audience   string
+		wantAuthOK bool
+	}{
+		{
+			name:       "accepts_dashboard_audience",
+			audience:   dashboardAudience,
+			wantAuthOK: true,
+		},
+		{
+			name:       "accepts_cli_audience",
+			audience:   cliAudience,
+			wantAuthOK: true,
+		},
+		{
+			name:       "rejects_unknown_audience",
+			audience:   "unknown-audience",
+			wantAuthOK: false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			jwtConfig := &JWTConfig{
+				Issuer:       issuer,
+				Audiences:    []string{dashboardAudience, cliAudience},
+				KeysLocation: jwksURL,
+			}
+			serverConfig := &Config{
+				HostKeyPEM: hostKey,
+				JWT:        jwtConfig,
+			}
+			server := New(serverConfig)
+			server.SetAllowRootLogin(true)
+
+			testUserHash, err := sshuserhash.HashUserID("test-user")
+			require.NoError(t, err)
+
+			currentUser := testutil.GetTestUsername(t)
+			authConfig := &sshauth.Config{
+				UserIDClaim:     sshauth.DefaultUserIDClaim,
+				AuthorizedUsers: []sshuserhash.UserIDHash{testUserHash},
+				MachineUsers: map[string][]uint32{
+					currentUser: {0},
+				},
+			}
+			server.UpdateSSHAuth(authConfig)
+
+			serverAddr := StartTestServer(t, server)
+			defer require.NoError(t, server.Stop())
+
+			host, portStr, err := net.SplitHostPort(serverAddr)
+			require.NoError(t, err)
+
+			token := generateValidJWT(t, privateKey, issuer, tc.audience)
+			config := &cryptossh.ClientConfig{
+				User: testutil.GetTestUsername(t),
+				Auth: []cryptossh.AuthMethod{
+					cryptossh.Password(token),
+				},
+				HostKeyCallback: cryptossh.InsecureIgnoreHostKey(),
+				Timeout:         2 * time.Second,
+			}
+
+			conn, err := cryptossh.Dial("tcp", net.JoinHostPort(host, portStr), config)
+			if tc.wantAuthOK {
+				require.NoError(t, err, "JWT authentication should succeed for audience %s", tc.audience)
+				defer func() {
+					if err := conn.Close(); err != nil {
+						t.Logf("close connection: %v", err)
+					}
+				}()
+
+				session, err := conn.NewSession()
+				require.NoError(t, err)
+				defer session.Close()
+
+				err = session.Shell()
+				require.NoError(t, err, "Shell should work with valid audience")
+			} else {
+				assert.Error(t, err, "JWT authentication should fail for unknown audience")
+			}
+		})
+	}
+}