CVE-2026-32871

Описание

FastMCP is a Pythonic way to build MCP servers and clients. Prior to version 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.

A flaw was found in FastMCP. An authenticated attacker can exploit a path traversal vulnerability in the _build_url() method of the RequestDirector class. By manipulating path parameters in an OpenAPI operation, an attacker can use directory traversal sequences (../) to bypass the intended API prefix. This allows the attacker to access arbitrary backend endpoints, leading to authenticated Server-Side Request Forgery (SSRF). Server-Side Request Forgery (SSRF) enables an attacker to compel the server-side application to send HTTP requests to an arbitrary domain.