Server-Side Request Forgery (SSRF)

Summary

FastMCP is a Pythonic way to build MCP servers and clients. All 2.x through 2.14.7 versions and 3.x versions prior to 3.2.0, the "OpenAPIProvider" in FastMCP exposes internal APIs to MCP clients by parsing OpenAPI specifications. The "RequestDirector" class is responsible for constructing HTTP requests to the backend service. A vulnerability exists in the "_build_url()" method. When an OpenAPI operation defines path parameters (e.g., /api/v1/users/{user_id}), the system directly substitutes parameter values into the URL template string without URL-encoding. Subsequently, "urllib.parse.urljoin()" resolves the final URL. Since "urljoin()" interprets "../" sequences as directory traversal, an attacker controlling a path parameter can perform path traversal attacks to escape the intended API prefix and access arbitrary backend endpoints. This results in authenticated SSRF, as requests are sent with the authorization headers configured in the MCP provider. This issue has been patched in version 3.2.0 and later.