feat(auth/generic): support custom introspection endpoints

docs/en/documentation/configuration/authentication/generic.md

@@ -104,10 +104,9 @@ When a request is received in this mode, the service will:
    - Verifies expiration (`exp`) and audience (`aud`).
    - Verifies required scopes in `scope` claim.
 4. For **Opaque Tokens**:
-   - Calls the introspection endpoint (as listed in the `authorizationServer`'s
-     OIDC configuration).
-   - Verifies that the token is `active`.
-   - Verifies expiration (`exp`) and audience (`aud`).
+   - Calls the introspection endpoint (either configured via `introspectionEndpoint`
+     or discovered from the `authorizationServer`'s OIDC configuration).
+   - Verifies expiration (`exp`) and audience (`aud` or `"audience"` fallback).
    - Verifies required scopes in `scope` field.
 
 #### Example
@@ -124,6 +123,22 @@ scopesRequired:
   - write
 ```
 
+#### Google Opaque Access Token Validation Example
+
+To use Google's `tokeninfo` endpoint for validating opaque access tokens, configure the service to use the `GET` method and `access_token` parameter name:
+
+```yaml
+kind: authServices
+name: google-auth
+type: generic
+audience: "YOUR_GOOGLE_CLIENT_ID.apps.googleusercontent.com"
+authorizationServer: https://accounts.google.com
+introspectionEndpoint: https://www.googleapis.com/oauth2/v1/tokeninfo
+introspectionMethod: GET
+introspectionParamName: access_token
+mcpEnabled: true
+```
+
 {{< notice tip >}} Use environment variable replacement with the format
 ${ENV_NAME} instead of hardcoding your secrets into the configuration file.
 {{< /notice >}}
@@ -142,3 +157,6 @@ ${ENV_NAME} instead of hardcoding your secrets into the configuration file.
 | authorizationServer |  string  |     true     | The base URL of your OIDC provider. The service will append `/.well-known/openid-configuration` to discover the JWKS URI. HTTP is allowed but logs a warning. |
 | mcpEnabled          |   bool   |    false     | Indicates if MCP endpoint authentication should be applied. Defaults to false.                                                                                |
 | scopesRequired      | []string |    false     | A list of required scopes that must be present in the token's `scope` claim to be considered valid.                                                           |
+| introspectionEndpoint| string  |    false     | Optional override for the token introspection URL. Useful if the provider does not list it in OIDC discovery (e.g., Google).                                  |
+| introspectionMethod  | string  |    false     | HTTP method to use for introspection. Defaults to "POST". Set to "GET" for providers like Google.                                                             |
+| introspectionParamName|string  |    false     | Parameter name for the token in the introspection request. Defaults to "token". Set to "access_token" for Google.                                             |

internal/auth/generic/generic.go

@@ -38,12 +38,15 @@ var _ auth.AuthServiceConfig = Config{}
 
 // Auth service configuration
 type Config struct {
-	Name                string   `yaml:"name" validate:"required"`
-	Type                string   `yaml:"type" validate:"required"`
-	Audience            string   `yaml:"audience" validate:"required"`
-	McpEnabled          bool     `yaml:"mcpEnabled"`
-	AuthorizationServer string   `yaml:"authorizationServer" validate:"required"`
-	ScopesRequired      []string `yaml:"scopesRequired"`
+	Name                   string   `yaml:"name" validate:"required"`
+	Type                   string   `yaml:"type" validate:"required"`
+	Audience               string   `yaml:"audience" validate:"required"`
+	McpEnabled             bool     `yaml:"mcpEnabled"`
+	AuthorizationServer    string   `yaml:"authorizationServer" validate:"required"`
+	ScopesRequired         []string `yaml:"scopesRequired"`
+	IntrospectionEndpoint  string   `yaml:"introspectionEndpoint"`
+	IntrospectionMethod    string   `yaml:"introspectionMethod"`
+	IntrospectionParamName string   `yaml:"introspectionParamName"`
 }
 
 // Returns the auth service type
@@ -61,6 +64,11 @@ func (cfg Config) Initialize() (auth.AuthService, error) {
 		return nil, fmt.Errorf("failed to discover OIDC config: %w", err)
 	}
 
+	// Override introspection URL if configured
+	if cfg.IntrospectionEndpoint != "" {
+		introspectionURL = cfg.IntrospectionEndpoint
+	}
+
 	// Create the keyfunc to fetch and cache the JWKS in the background
 	kf, err := keyfunc.NewDefault([]string{jwksURL})
 	if err != nil {
@@ -288,14 +296,33 @@ func (a AuthService) validateOpaqueToken(ctx context.Context, tokenStr string) e
 		}
 	}
 
-	data := url.Values{}
-	data.Set("token", tokenStr)
+	paramName := a.IntrospectionParamName
+	if paramName == "" {
+		paramName = "token"
+	}
 
-	req, err := http.NewRequestWithContext(ctx, "POST", introspectionURL, strings.NewReader(data.Encode()))
-	if err != nil {
-		return fmt.Errorf("failed to create introspection request: %w", err)
+	var req *http.Request
+	if a.IntrospectionMethod == "GET" {
+		u, err := url.Parse(introspectionURL)
+		if err != nil {
+			return fmt.Errorf("failed to parse introspection URL: %w", err)
+		}
+		q := u.Query()
+		q.Set(paramName, tokenStr)
+		u.RawQuery = q.Encode()
+		req, err = http.NewRequestWithContext(ctx, "GET", u.String(), nil)
+		if err != nil {
+			return fmt.Errorf("failed to create introspection request: %w", err)
+		}
+	} else {
+		data := url.Values{}
+		data.Set(paramName, tokenStr)
+		req, err = http.NewRequestWithContext(ctx, "POST", introspectionURL, strings.NewReader(data.Encode()))
+		if err != nil {
+			return fmt.Errorf("failed to create introspection request: %w", err)
+		}
+		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	}
-	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("Accept", "application/json")
 
 	// Send request to auth server's introspection endpoint
@@ -317,17 +344,18 @@ func (a AuthService) validateOpaqueToken(ctx context.Context, tokenStr string) e
 	}
 
 	var introspectResp struct {
-		Active bool            `json:"active"`
-		Scope  string          `json:"scope"`
-		Aud    json.RawMessage `json:"aud"`
-		Exp    int64           `json:"exp"`
+		Active   *bool           `json:"active"`
+		Scope    string          `json:"scope"`
+		Aud      json.RawMessage `json:"aud"`
+		Audience json.RawMessage `json:"audience"`
+		Exp      int64           `json:"exp"`
 	}
 
 	if err := json.Unmarshal(body, &introspectResp); err != nil {
 		return fmt.Errorf("failed to parse introspection response: %w", err)
 	}
 
-	if !introspectResp.Active {
+	if introspectResp.Active != nil && !*introspectResp.Active {
 		logger.InfoContext(ctx, "token is not active")
 		return &MCPAuthError{Code: http.StatusUnauthorized, Message: "token is not active", ScopesRequired: a.ScopesRequired}
 	}
@@ -341,16 +369,22 @@ func (a AuthService) validateOpaqueToken(ctx context.Context, tokenStr string) e
 
 	// Extract audience
 	// According to RFC 7662, the aud claim can be a string or an array of strings
+	// Fallback to "audience" for Google tokeninfo
+	audData := introspectResp.Aud
+	if len(audData) == 0 {
+		audData = introspectResp.Audience
+	}
+
 	var aud []string
-	if len(introspectResp.Aud) > 0 {
+	if len(audData) > 0 {
 		var audStr string
 		var audArr []string
-		if err := json.Unmarshal(introspectResp.Aud, &audStr); err == nil {
+		if err := json.Unmarshal(audData, &audStr); err == nil {
 			aud = []string{audStr}
-		} else if err := json.Unmarshal(introspectResp.Aud, &audArr); err == nil {
+		} else if err := json.Unmarshal(audData, &audArr); err == nil {
 			aud = audArr
 		} else {
-			logger.WarnContext(ctx, "failed to parse aud claim in introspection response")
+			logger.WarnContext(ctx, "failed to parse aud or audience claim in introspection response")
 			return &MCPAuthError{Code: http.StatusUnauthorized, Message: "invalid aud claim", ScopesRequired: a.ScopesRequired}
 		}
 	}

tests/auth/auth_integration_test.go

@@ -29,6 +29,7 @@ import (
 
 	"github.com/MicahParks/jwkset"
 	"github.com/golang-jwt/jwt/v5"
+	"github.com/googleapis/mcp-toolbox/internal/sources"
 	"github.com/googleapis/mcp-toolbox/internal/testutils"
 	"github.com/googleapis/mcp-toolbox/tests"
 )
@@ -191,3 +192,86 @@ func TestMcpAuth(t *testing.T) {
 		})
 	}
 }
+
+// TestGoogleTokenValidation tests validation of Google access token
+func TestGoogleTokenValidation(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
+	defer cancel()
+
+	// Get access token
+	accessToken, err := sources.GetIAMAccessToken(ctx)
+	if err != nil {
+		t.Errorf("error getting access token from ADC: %s", err)
+	}
+
+	// Call tokeninfo to get audience
+	resp, err := http.Get("https://www.googleapis.com/oauth2/v1/tokeninfo?access_token=" + accessToken)
+	if err != nil {
+		t.Fatalf("failed to call tokeninfo: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp.Body)
+		t.Fatalf("tokeninfo returned non-200 status %d: %s", resp.StatusCode, string(body))
+	}
+
+	var tokenInfo struct {
+		Audience string `json:"audience"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&tokenInfo); err != nil {
+		t.Fatalf("failed to decode tokeninfo response: %v", err)
+	}
+
+	aud := tokenInfo.Audience
+	if aud == "" {
+		t.Fatalf("audience is empty in tokeninfo response")
+	}
+
+	toolsFile := map[string]any{
+		"sources": map[string]any{},
+		"authServices": map[string]any{
+			"google-auth": map[string]any{
+				"type":                   "generic",
+				"audience":               aud,
+				"authorizationServer":    "https://accounts.google.com",
+				"introspectionEndpoint":  "https://www.googleapis.com/oauth2/v1/tokeninfo",
+				"introspectionMethod":    "GET",
+				"introspectionParamName": "access_token",
+				"mcpEnabled":             true,
+			},
+		},
+		"tools": map[string]any{},
+	}
+
+	args := []string{"--enable-api", "--toolbox-url=http://127.0.0.1:5005", "--port=5005"}
+	cmd, cleanup, err := tests.StartCmd(ctx, toolsFile, args...)
+	if err != nil {
+		t.Fatalf("command initialization returned an error: %s", err)
+	}
+	defer cleanup()
+
+	waitCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+	out, err := testutils.WaitForString(waitCtx, regexp.MustCompile(`Server ready to serve`), cmd.Out)
+	if err != nil {
+		t.Logf("toolbox command logs: \n%s", out)
+		t.Fatalf("toolbox didn't start successfully: %s", err)
+	}
+
+	api := "http://127.0.0.1:5005/mcp/sse"
+
+	req, _ := http.NewRequest(http.MethodGet, api, nil)
+	req.Header.Add("Authorization", "Bearer "+accessToken)
+
+	resp, err = http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("unable to send request: %s", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		bodyBytes, _ := io.ReadAll(resp.Body)
+		t.Fatalf("expected 200, got %d: %s", resp.StatusCode, string(bodyBytes))
+	}
+}