auth/extauth: only forward an ID-JAG to the MCP authorization server

auth/extauth/enterprise_handler.go

@@ -195,6 +195,13 @@ func (h *EnterpriseHandler) Authorize(ctx context.Context, req *http.Request, re
 	if err != nil {
 		return fmt.Errorf("token exchange failed: %w", err)
 	}
+	// The token is sent on to the MCP Server's authorization server, a different
+	// party than the IdP. Only forward it if the IdP says it is an ID-JAG, so
+	// that any other token the IdP issued, such as an access token, does not
+	// leave it.
+	if issued, _ := idJAGToken.Extra("issued_token_type").(string); issued != oauthex.TokenTypeIDJAG {
+		return fmt.Errorf("token exchange issued a token of type %q, want %q", issued, oauthex.TokenTypeIDJAG)
+	}
 
 	// Step 4: Discover MCP Server token endpoint
 	mcpMeta, err := auth.GetAuthServerMetadata(ctx, h.config.MCPAuthServerURL, httpClient)

auth/extauth/enterprise_handler_test.go

@@ -303,6 +303,62 @@ func TestEnterpriseHandler_ConcurrentAuthorize(t *testing.T) {
 	wg.Wait()
 }
 
+// TestEnterpriseHandler_Authorize_NotIDJAG checks that a token the IdP did not
+// issue as an ID-JAG is not sent on to the MCP Server's authorization server,
+// even one that would accept it.
+func TestEnterpriseHandler_Authorize_NotIDJAG(t *testing.T) {
+	const idpAccessToken = "idp-access-token"
+
+	idpServer := oauthtest.NewFakeIdPServer(oauthtest.IdPConfig{
+		PreregisteredClients: map[string]oauthtest.ClientInfo{
+			"idp_client_id": {Secret: "idp_secret"},
+		},
+		TokenExchangeConfig: &oauthtest.TokenExchangeConfig{
+			IDJAGToken:      idpAccessToken,
+			IssuedTokenType: "urn:ietf:params:oauth:token-type:access_token",
+		},
+	})
+	idpServer.Start(t)
+
+	mcpAuthServer := oauthtest.NewFakeAuthorizationServer(oauthtest.Config{
+		RegistrationConfig: &oauthtest.RegistrationConfig{
+			PreregisteredClients: map[string]oauthtest.ClientInfo{
+				"mcp_client_id": {Secret: "mcp_secret"},
+			},
+		},
+		JWTBearerConfig: &oauthtest.JWTBearerConfig{
+			ValidAssertions: []string{idpAccessToken},
+		},
+	})
+	mcpAuthServer.Start(t)
+
+	config := validEnterpriseHandlerConfig()
+	config.IdPIssuerURL = idpServer.URL()
+	config.IdPCredentials.ClientSecretAuth = &oauthex.ClientSecretAuth{ClientSecret: "idp_secret"}
+	config.MCPAuthServerURL = mcpAuthServer.URL()
+	config.MCPCredentials.ClientSecretAuth = &oauthex.ClientSecretAuth{ClientSecret: "mcp_secret"}
+
+	handler, err := NewEnterpriseHandler(config)
+	if err != nil {
+		t.Fatalf("NewEnterpriseHandler failed: %v", err)
+	}
+
+	req := httptest.NewRequest(http.MethodGet, "https://mcp.example.com/api", nil)
+	resp := &http.Response{
+		StatusCode: http.StatusUnauthorized,
+		Header:     make(http.Header),
+		Body:       http.NoBody,
+		Request:    req,
+	}
+	err = handler.Authorize(context.Background(), req, resp)
+	if err == nil || !strings.Contains(err.Error(), "token of type") {
+		t.Fatalf("Authorize error = %v, want an issued token type error", err)
+	}
+	if ts, _ := handler.TokenSource(context.Background()); ts != nil {
+		t.Error("TokenSource is set after the token exchange issued a token that is not an ID-JAG")
+	}
+}
+
 func TestEnterpriseHandler_Authorize_IDTokenFetcherError(t *testing.T) {
 	config := validEnterpriseHandlerConfig()
 	config.IDTokenFetcher = func(ctx context.Context) (*oauth2.Token, error) {

internal/oauthtest/fake_idp_server.go

@@ -25,6 +25,10 @@ type TokenExchangeConfig struct {
 	// IDJAGToken is the ID-JAG value returned from token exchange.
 	// Defaults to "test-id-jag-token" if empty.
 	IDJAGToken string
+
+	// IssuedTokenType is the issued_token_type returned from token exchange.
+	// Defaults to oauthex.TokenTypeIDJAG if empty.
+	IssuedTokenType string
 }
 
 // IdPConfig holds configuration for [FakeIdPServer].
@@ -218,11 +222,15 @@ func (s *FakeIdPServer) handleTokenExchangeGrant(w http.ResponseWriter, r *http.
 	if idJAG == "" {
 		idJAG = "test-id-jag-token"
 	}
+	issuedTokenType := s.config.TokenExchangeConfig.IssuedTokenType
+	if issuedTokenType == "" {
+		issuedTokenType = oauthex.TokenTypeIDJAG
+	}
 
 	w.Header().Set("Content-Type", "application/json")
 	json.NewEncoder(w).Encode(map[string]any{
 		"access_token":      idJAG,
-		"issued_token_type": oauthex.TokenTypeIDJAG,
+		"issued_token_type": issuedTokenType,
 		"token_type":        "N_A",
 		"expires_in":        300,
 	})