fix: federated client credentials not working if sub ≠ client_id (#1342)

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/internal/service/oidc_service.go

@@ -1644,34 +1644,19 @@ func clientAuthCredentialsFromCreateTokensDto(d *dto.OidcCreateTokensDto) Client
 }
 
 func (s *OidcService) verifyClientCredentialsInternal(ctx context.Context, tx *gorm.DB, input ClientAuthCredentials, allowPublicClientsWithoutAuth bool) (client *model.OidcClient, err error) {
-	isClientAssertion := input.ClientAssertionType == ClientAssertionTypeJWTBearer && input.ClientAssertion != ""
-
-	// Determine the client ID based on the authentication method
-	var clientID string
-	switch {
-	case isClientAssertion:
-		// Extract client ID from the JWT assertion's 'sub' claim
-		clientID, err = s.extractClientIDFromAssertion(input.ClientAssertion)
-		if err != nil {
-			slog.Error("Failed to extract client ID from assertion", "error", err)
-			return nil, &common.OidcClientAssertionInvalidError{}
-		}
-	case input.ClientID != "":
-		// Use the provided client ID for other authentication methods
-		clientID = input.ClientID
-	default:
+	if input.ClientID == "" {
 		return nil, &common.OidcMissingClientCredentialsError{}
 	}
 
 	// Load the OIDC client's configuration
 	err = tx.
 		WithContext(ctx).
-		First(&client, "id = ?", clientID).
+		First(&client, "id = ?", input.ClientID).
 		Error
-	if err != nil {
-		if errors.Is(err, gorm.ErrRecordNotFound) && isClientAssertion {
-			return nil, &common.OidcClientAssertionInvalidError{}
-		}
+	if errors.Is(err, gorm.ErrRecordNotFound) {
+		slog.WarnContext(ctx, "Client not found", slog.String("client", input.ClientID))
+		return nil, &common.OidcClientNotFoundError{}
+	} else if err != nil {
 		return nil, err
 	}
 
@@ -1686,7 +1671,7 @@ func (s *OidcService) verifyClientCredentialsInternal(ctx context.Context, tx *g
 		return client, nil
 
 	// Next, check if we want to use client assertions from federated identities
-	case isClientAssertion:
+	case input.ClientAssertionType == ClientAssertionTypeJWTBearer && input.ClientAssertion != "":
 		err = s.verifyClientAssertionFromFederatedIdentities(ctx, client, input)
 		if err != nil {
 			slog.WarnContext(ctx, "Invalid assertion for client", slog.String("client", client.ID), slog.Any("error", err))
@@ -1783,36 +1768,20 @@ func (s *OidcService) verifyClientAssertionFromFederatedIdentities(ctx context.C
 	// (Note: we don't use jwt.WithIssuer() because that would be redundant)
 	_, err = jwt.Parse(assertion,
 		jwt.WithValidate(true),
+
 		jwt.WithAcceptableSkew(clockSkew),
 		jwt.WithKeySet(jwks, jws.WithInferAlgorithmFromKey(true), jws.WithUseDefault(true)),
 		jwt.WithAudience(audience),
 		jwt.WithSubject(subject),
 	)
 	if err != nil {
-		return fmt.Errorf("client assertion is not valid: %w", err)
+		return fmt.Errorf("client assertion could not be verified: %w", err)
 	}
 
 	// If we're here, the assertion is valid
 	return nil
 }
 
-// extractClientIDFromAssertion extracts the client_id from the JWT assertion's 'sub' claim
-func (s *OidcService) extractClientIDFromAssertion(assertion string) (string, error) {
-	// Parse the JWT without verification first to get the claims
-	insecureToken, err := jwt.ParseInsecure([]byte(assertion))
-	if err != nil {
-		return "", fmt.Errorf("failed to parse JWT assertion: %w", err)
-	}
-
-	// Extract the subject claim which must be the client_id according to RFC 7523
-	sub, ok := insecureToken.Subject()
-	if !ok || sub == "" {
-		return "", fmt.Errorf("missing or invalid 'sub' claim in JWT assertion")
-	}
-
-	return sub, nil
-}
-
 func (s *OidcService) GetClientPreview(ctx context.Context, clientID string, userID string, scopes []string) (*dto.OidcClientPreviewDto, error) {
 	tx := s.db.Begin()
 	defer func() {

backend/internal/service/oidc_service_test.go

@@ -229,6 +229,12 @@ func TestOidcService_verifyClientCredentialsInternal(t *testing.T) {
 					Subject:  federatedClient.ID,
 					JWKS:     federatedClientIssuer + "/jwks.json",
 				},
+				{
+					Issuer:   "federated-issuer-2",
+					Audience: federatedClientAudience,
+					Subject:  "my-federated-client",
+					JWKS:     federatedClientIssuer + "/jwks.json",
+				},
 				{Issuer: federatedClientIssuerDefaults},
 			},
 		},
@@ -461,6 +467,43 @@ func TestOidcService_verifyClientCredentialsInternal(t *testing.T) {
 
 				// Generate a token
 				input := dto.OidcCreateTokensDto{
+					ClientID:            federatedClient.ID,
+					ClientAssertion:     string(signedToken),
+					ClientAssertionType: ClientAssertionTypeJWTBearer,
+				}
+				createdToken, err := s.createTokenFromClientCredentials(t.Context(), input)
+				require.NoError(t, err)
+				require.NotNil(t, token)
+
+				// Verify the token
+				claims, err := s.jwtService.VerifyOAuthAccessToken(createdToken.AccessToken)
+				require.NoError(t, err, "Failed to verify generated token")
+
+				// Check the claims
+				subject, ok := claims.Subject()
+				_ = assert.True(t, ok, "User ID not found in token") &&
+					assert.Equal(t, "client-"+federatedClient.ID, subject, "Token subject should match federated client ID with prefix")
+				audience, ok := claims.Audience()
+				_ = assert.True(t, ok, "Audience not found in token") &&
+					assert.Equal(t, []string{federatedClient.ID}, audience, "Audience should contain the federated client ID")
+			})
+
+			t.Run("Succeeds with valid assertion and custom subject", func(t *testing.T) {
+				// Create JWT for federated identity
+				token, err := jwt.NewBuilder().
+					Issuer("federated-issuer-2").
+					Audience([]string{federatedClientAudience}).
+					Subject("my-federated-client").
+					IssuedAt(time.Now()).
+					Expiration(time.Now().Add(10 * time.Minute)).
+					Build()
+				require.NoError(t, err)
+				signedToken, err := jwt.Sign(token, jwt.WithKey(jwa.ES256(), privateJWK))
+				require.NoError(t, err)
+
+				// Generate a token
+				input := dto.OidcCreateTokensDto{
+					ClientID:            federatedClient.ID,
 					ClientAssertion:     string(signedToken),
 					ClientAssertionType: ClientAssertionTypeJWTBearer,
 				}
@@ -483,6 +526,7 @@ func TestOidcService_verifyClientCredentialsInternal(t *testing.T) {
 
 			t.Run("Fails with invalid assertion", func(t *testing.T) {
 				input := dto.OidcCreateTokensDto{
+					ClientID:            confidentialClient.ID,
 					ClientAssertion:     "invalid.jwt.token",
 					ClientAssertionType: ClientAssertionTypeJWTBearer,
 				}