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* OAuth Authentication Middleware
*
* Validates Bearer tokens on MCP endpoints and sets up the token context
* for per-request GitLab API access.
*
* This middleware:
* 1. Extracts and validates the Bearer token from Authorization header
* 2. Verifies the JWT signature and expiration
* 3. Loads the associated session
* 4. Refreshes GitLab token if needed
* 5. Sets up token context for the request
*/
import { Request, Response, NextFunction, RequestHandler } from 'express';
import { loadOAuthConfig } from '../oauth/config';
import { sessionStore } from '../oauth/session-store';
import { isPastAgeLimit } from '../oauth/storage/types';
import { verifyMCPToken } from '../oauth/token-utils';
import { withFreshGitLabToken, GitLabGrantRevokedError } from '../oauth/gitlab-token-refresh';
import { getBaseUrl } from '../oauth/endpoints/metadata';
import {
MCP_SCOPES,
isProtectedResource,
resourceForPath,
resourceMetadataUrl,
} from '../oauth/resource';
import { logWarn, logError, logDebug, truncateId } from '../logger';
import { MCPTokenPayload, OAuthErrorResponse } from '../oauth/types';
import { getMinimalRequestContext } from '../utils/request-logger';
import { GITLAB_BASE_URL } from '../config';
/**
* OAuth authentication middleware for Express
*
* Apply this middleware to routes that require OAuth authentication.
* It validates the Bearer token and sets up the token context.
*
* @param req - Express request
* @param res - Express response
* @param next - Express next function
*/
export async function oauthAuthMiddleware(
req: Request,
res: Response,
next: NextFunction,
): Promise<void> {
const reject = await authenticateRequest(req, res);
if (reject) reject();
else next();
}
/** Authenticate once; defer error responses until the caller has applied its quota.
* A signed token still needs a session lookup to detect revocation before choosing its quota.
*/
async function authenticateRequest(req: Request, res: Response): Promise<(() => void) | undefined> {
const config = loadOAuthConfig();
if (!config) {
return () => sendUnauthorized(req, res, 'server_error', 'OAuth not configured');
}
// Extract Bearer token from Authorization header
const authHeader = req.headers.authorization;
if (!authHeader) {
return () => sendUnauthorized(req, res, 'unauthorized', 'Missing Authorization header');
}
if (!authHeader.startsWith('Bearer ')) {
return () =>
sendUnauthorized(
req,
res,
'unauthorized',
'Invalid Authorization header format. Expected: Bearer <token>',
);
}
const token = authHeader.slice(7); // Remove "Bearer " prefix
if (!token) {
return () => sendUnauthorized(req, res, 'unauthorized', 'Empty Bearer token');
}
// Verify JWT token
const payload = verifyMCPToken(token, config.sessionSecret);
if (!payload) {
return () => sendUnauthorized(req, res, 'invalid_token', 'Token is invalid or expired');
}
if (!isIssuedForThisServer(config.issuer, payload)) {
return () =>
sendUnauthorized(req, res, 'invalid_token', 'Token was not issued for this server');
}
// Get session from token
const sessionId = payload.sid;
let session;
try {
session = await sessionStore.getSession(sessionId);
} catch (error: unknown) {
// Storage outage: the credentials cannot be verified, which is not a bad token.
logError('Failed to load OAuth session', { err: error as Error });
return () => {
res.status(503).json({
error: 'temporarily_unavailable',
error_description: 'Session storage is unavailable',
});
};
}
// A session past its age limit stays stored until cleanup ends its grants, and an access
// token can outlive that limit; it authenticates nothing either way.
if (!session || isPastAgeLimit(session)) {
return () => sendUnauthorized(req, res, 'invalid_token', 'Session not found or expired');
}
// Verify token matches session
if (session.mcpAccessToken !== token) {
// Token might have been rotated
return () => sendUnauthorized(req, res, 'invalid_token', 'Token has been superseded');
}
if (!hasGrantedScope(payload.scope, session.scopes)) {
return () => sendInsufficientScope(req, res);
}
// Refresh GitLab token if it's expiring soon (5 minute buffer)
let updatedSession;
try {
updatedSession = await withFreshGitLabToken(session, config);
} catch (error: unknown) {
logError('Failed to refresh GitLab token during request', { err: error as Error });
if (error instanceof GitLabGrantRevokedError) {
return () =>
sendUnauthorized(
req,
res,
'invalid_token',
'GitLab token refresh failed. Please re-authenticate.',
);
} else {
// A GitLab outage is not a bad token: the account stays linked, the client retries.
return () => {
res.status(503).json({
error: 'temporarily_unavailable',
error_description: 'GitLab is temporarily unavailable',
});
};
}
}
if (!updatedSession) {
return () => sendUnauthorized(req, res, 'invalid_token', 'Session lost during token refresh');
}
// Store OAuth session info in res.locals for route handlers
// This is used by:
// 1. Transport handlers to associate MCP sessions with OAuth sessions
// 2. Route handlers to set up token context around transport.handleRequest()
//
// NOTE: We do NOT use runWithTokenContext here because middleware's next() chain
// breaks AsyncLocalStorage propagation to MCP SDK's internal handlers.
// Instead, route handlers must wrap transport.handleRequest() with runWithTokenContext()
// using the data stored here.
res.locals.oauthSessionId = updatedSession.id;
res.locals.gitlabToken = updatedSession.gitlabAccessToken;
res.locals.gitlabUserId = updatedSession.gitlabUserId;
res.locals.gitlabUsername = updatedSession.gitlabUsername;
res.locals.gitlabScopes = updatedSession.gitlabScopes;
// Multi-instance support: use session's API URL or fallback to global config
res.locals.gitlabApiUrl = updatedSession.gitlabApiUrl ?? GITLAB_BASE_URL;
res.locals.instanceLabel = updatedSession.instanceLabel;
res.locals.mcpResource = resourceForPath(config.issuer, req.path);
// Tool dispatch requires mcp:tools among them.
res.locals.mcpScopes = tokenScopes(payload.scope);
logDebug('OAuth session validated, passing to route handler', {
sessionId: truncateId(updatedSession.id),
method: req.method,
path: req.path,
});
return undefined;
}
/**
* Create OAuth middleware for specific routes
*
* Apply the request quota after OAuth classification, including rejected requests.
* A live transport ID does not authenticate a caller; the quota uses verified OAuth locals.
*/
export function createOAuthMiddleware(rateLimit: RequestHandler): RequestHandler {
return async (req, res, next): Promise<void> => {
const reject = await authenticateRequest(req, res);
if (reject) return rateLimit(req, res, reject);
return rateLimit(req, res, next);
};
}
/**
* Optional OAuth middleware
*
* Like oauthAuthMiddleware, but doesn't require authentication.
* If a valid token is provided, sets up res.locals with session info.
* If no token or invalid token, continues without setting res.locals.
*
* Useful for endpoints that work with or without authentication.
*/
export async function optionalOAuthMiddleware(
req: Request,
res: Response,
next: NextFunction,
): Promise<void> {
const config = loadOAuthConfig();
if (!config) {
// OAuth not configured, continue without context
next();
return;
}
const authHeader = req.headers.authorization;
if (!authHeader?.startsWith('Bearer ')) {
// No token provided, continue without context
next();
return;
}
const token = authHeader.slice(7);
if (!token) {
next();
return;
}
// Try to validate token
const payload = verifyMCPToken(token, config.sessionSecret);
if (!payload || !isIssuedForThisServer(config.issuer, payload)) {
// Invalid token, but this is optional auth, so continue
next();
return;
}
let session;
try {
session = await sessionStore.getSession(payload.sid);
} catch (error: unknown) {
// Optional auth: an unreadable session store means no authenticated context.
logError('Failed to load OAuth session', { err: error as Error });
next();
return;
}
if (
session?.mcpAccessToken !== token ||
isPastAgeLimit(session) ||
!hasGrantedScope(payload.scope, session.scopes)
) {
next();
return;
}
// Valid token - store session info in res.locals for route handler
res.locals.oauthSessionId = session.id;
res.locals.gitlabToken = session.gitlabAccessToken;
res.locals.gitlabUserId = session.gitlabUserId;
res.locals.gitlabUsername = session.gitlabUsername;
res.locals.gitlabScopes = session.gitlabScopes;
// Multi-instance support: use session's API URL or fallback to global config
res.locals.gitlabApiUrl = session.gitlabApiUrl ?? GITLAB_BASE_URL;
res.locals.instanceLabel = session.instanceLabel;
res.locals.mcpResource = resourceForPath(config.issuer, req.path);
res.locals.mcpScopes = tokenScopes(payload.scope);
next();
}
/**
* `iss` is our issuer and `aud` one of our resources (RFC 9068 section 4). Either resource
* is accepted on either path on purpose: the root and `/mcp` are the same server with the
* same tools and grants, so audience binding guards against tokens of other resource
* servers, not between these two. Clients also legitimately cross them: ChatGPT requests
* tokens for the root resource and calls `/mcp`.
*/
function isIssuedForThisServer(issuer: string, payload: MCPTokenPayload): boolean {
return payload.iss === issuer && isProtectedResource(issuer, payload.aud);
}
/**
* The token carries at least one MCP scope and nothing beyond the session's grant, so a
* token minted before a narrower grant cannot outlive it.
*/
function tokenScopes(tokenScope: string): string[] {
return tokenScope.split(' ').filter(Boolean);
}
function hasGrantedScope(tokenScope: string, grantedScopes: string[]): boolean {
const scopes = tokenScopes(tokenScope);
return (
scopes.some((scope) => MCP_SCOPES.includes(scope)) &&
scopes.every((scope) => grantedScopes.includes(scope))
);
}
/** RFC 6750 section 3.1: a valid token without the needed scope gets 403. */
function sendInsufficientScope(req: Request, res: Response): void {
logWarn('Authentication rejected', {
event: 'auth_rejected',
...getMinimalRequestContext(req),
reason: 'insufficient_scope',
});
res.setHeader(
'WWW-Authenticate',
`Bearer realm="gitlab-mcp", error="insufficient_scope", scope="${MCP_SCOPES.join(' ')}"`,
);
const response: OAuthErrorResponse = {
error: 'insufficient_scope',
error_description: 'Token lacks the scope this server requires',
};
res.status(403).json(response);
}
/**
* Send unauthorized response with OAuth error format
*
* Includes WWW-Authenticate header with resource parameter (RFC 9470)
* to help clients discover the authorization server.
*
* Also logs the auth rejection for debugging and security monitoring.
*/
function sendUnauthorized(req: Request, res: Response, error: string, description: string): void {
// Log auth rejection with structured context
logWarn('Authentication rejected', {
event: 'auth_rejected',
...getMinimalRequestContext(req),
reason: error,
description,
});
const response: OAuthErrorResponse = {
error,
error_description: description,
};
// resource_metadata points at the metadata of the endpoint that was called (RFC 9728
// section 5.1), built from the configured issuer rather than request headers.
const config = loadOAuthConfig();
const metadataUrl = config
? resourceMetadataUrl(resourceForPath(config.issuer, req.path))
: `${getBaseUrl(req)}/.well-known/oauth-protected-resource`;
// Set WWW-Authenticate header with resource_metadata parameter
// Points to Protected Resource Metadata document per MCP spec. A rejected token also
// names the error (RFC 6750 section 3); a request without credentials does not
// (RFC 6750 section 3.1).
const tokenError =
error === 'invalid_token' ? `, error="invalid_token", error_description="${description}"` : '';
res.setHeader(
'WWW-Authenticate',
`Bearer realm="gitlab-mcp", resource_metadata="${metadataUrl}"${tokenError}`,
);
res.status(401).json(response);
}
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