[client] Force interactive login when extending the auth session

A session extend must be answered from the account the peer is registered
under. With a silent PKCE flow (DisablePromptLogin or max_age=0) the IdP
answers from whatever session it already holds, which need not be the
peer's account when several are signed in; the token then fails the
user match in ExtendAuthSession with no way to pick another account.

Mark the PKCE flow request as a session extend so the management server
can force prompt=login for it, overriding the configured silent flow.

shared/management/client/client.go

@@ -21,7 +21,7 @@ type Client interface {
 	// is not eligible for session extension.
 	ExtendAuthSession(sysInfo *system.Info, jwtToken string) (*proto.ExtendAuthSessionResponse, error)
 	GetDeviceAuthorizationFlow() (*proto.DeviceAuthorizationFlow, error)
-	GetPKCEAuthorizationFlow() (*proto.PKCEAuthorizationFlow, error)
+	GetPKCEAuthorizationFlow(sessionExtend bool) (*proto.PKCEAuthorizationFlow, error)
 	GetServerURL() string
 	// IsHealthy returns the current connection status without blocking.
 	// Used by the engine to monitor connectivity in the background.

shared/management/client/client_test.go

@@ -595,7 +595,12 @@ func Test_GetPKCEAuthorizationFlow(t *testing.T) {
 		},
 	}
 
+	var gotRequest mgmtProto.PKCEAuthorizationFlowRequest
 	mgmtMockServer.GetPKCEAuthorizationFlowFunc = func(ctx context.Context, req *mgmtProto.EncryptedMessage) (*mgmtProto.EncryptedMessage, error) {
+		if err := encryption.DecryptMessage(client.key.PublicKey(), serverKey, req.Body, &gotRequest); err != nil {
+			return nil, err
+		}
+
 		encryptedResp, err := encryption.EncryptMessage(client.key.PublicKey(), serverKey, expectedFlowInfo)
 		if err != nil {
 			return nil, err
@@ -608,11 +613,13 @@ func Test_GetPKCEAuthorizationFlow(t *testing.T) {
 		}, nil
 	}
 
-	flowInfo, err := client.GetPKCEAuthorizationFlow()
+	flowInfo, err := client.GetPKCEAuthorizationFlow(true)
 	if err != nil {
 		t.Error("error while retrieving pkce auth flow information")
 	}
 
+	assert.True(t, gotRequest.GetSessionExtend(), "session extend should reach the server")
+
 	assert.Equal(t, expectedFlowInfo.ProviderConfig.ClientID, flowInfo.ProviderConfig.ClientID, "provider configured client ID should match")
 	assert.Equal(t, expectedFlowInfo.ProviderConfig.ClientSecret, flowInfo.ProviderConfig.ClientSecret, "provider configured client secret should match") //nolint:staticcheck
 }

shared/management/client/grpc.go

@@ -701,7 +701,11 @@ func (c *GrpcClient) GetDeviceAuthorizationFlow() (*proto.DeviceAuthorizationFlo
 
 // GetPKCEAuthorizationFlow returns a pkce authorization flow information.
 // It also takes care of encrypting and decrypting messages.
-func (c *GrpcClient) GetPKCEAuthorizationFlow() (*proto.PKCEAuthorizationFlow, error) {
+//
+// sessionExtend tells the server the flow will renew an existing peer's session
+// rather than log one in, so it can rule out a configuration that would let the
+// IdP answer from an unrelated account. See PKCEAuthorizationFlowRequest.
+func (c *GrpcClient) GetPKCEAuthorizationFlow(sessionExtend bool) (*proto.PKCEAuthorizationFlow, error) {
 	if !c.ready() {
 		return nil, fmt.Errorf("no connection to management in order to get pkce authorization flow")
 	}
@@ -714,7 +718,7 @@ func (c *GrpcClient) GetPKCEAuthorizationFlow() (*proto.PKCEAuthorizationFlow, e
 	mgmCtx, cancel := context.WithTimeout(c.ctx, time.Second*2)
 	defer cancel()
 
-	message := &proto.PKCEAuthorizationFlowRequest{}
+	message := &proto.PKCEAuthorizationFlowRequest{SessionExtend: sessionExtend}
 	encryptedMSG, err := encryption.EncryptMessage(*serverKey, c.key, message)
 	if err != nil {
 		return nil, err

shared/management/client/mock.go

@@ -16,7 +16,7 @@ type MockClient struct {
 	LoginFunc                      func(info *system.Info, sshKey []byte, dnsLabels domain.List) (*proto.LoginResponse, error)
 	ExtendAuthSessionFunc          func(info *system.Info, jwtToken string) (*proto.ExtendAuthSessionResponse, error)
 	GetDeviceAuthorizationFlowFunc func() (*proto.DeviceAuthorizationFlow, error)
-	GetPKCEAuthorizationFlowFunc   func() (*proto.PKCEAuthorizationFlow, error)
+	GetPKCEAuthorizationFlowFunc   func(sessionExtend bool) (*proto.PKCEAuthorizationFlow, error)
 	GetServerURLFunc               func() string
 	HealthCheckFunc                func() error
 	SyncMetaFunc                   func(sysInfo *system.Info) error
@@ -80,11 +80,11 @@ func (m *MockClient) GetDeviceAuthorizationFlow() (*proto.DeviceAuthorizationFlo
 	return m.GetDeviceAuthorizationFlowFunc()
 }
 
-func (m *MockClient) GetPKCEAuthorizationFlow() (*proto.PKCEAuthorizationFlow, error) {
+func (m *MockClient) GetPKCEAuthorizationFlow(sessionExtend bool) (*proto.PKCEAuthorizationFlow, error) {
 	if m.GetPKCEAuthorizationFlowFunc == nil {
 		return nil, nil
 	}
-	return m.GetPKCEAuthorizationFlowFunc()
+	return m.GetPKCEAuthorizationFlowFunc(sessionExtend)
 }
 
 func (m *MockClient) HealthCheck() error {

shared/management/proto/management.proto

@@ -530,8 +530,18 @@ message DeviceAuthorizationFlow {
   }
 }
 
-// PKCEAuthorizationFlowRequest empty struct for future expansion
-message PKCEAuthorizationFlowRequest {}
+// PKCEAuthorizationFlowRequest asks for the PKCE flow configuration to use for
+// an upcoming authorization request.
+message PKCEAuthorizationFlowRequest {
+  // SessionExtend indicates the flow will renew the SSO session of a peer that
+  // is already registered, rather than log in or register one. An extend is
+  // bound to the account that peer belongs to, so the server must not answer it
+  // with a configuration that lets the IdP reply from whatever session is
+  // already active: with several accounts signed in at the IdP that need not be
+  // the peer's own, and the resulting token is rejected as a peer/user mismatch
+  // with no way for the user to correct it.
+  bool SessionExtend = 1;
+}
 
 // PKCEAuthorizationFlow represents Authorization Code Flow information
 // that can be used by the client to login initiate a Oauth 2.0 authorization code grant flow