CVE-2026-32871
CriticalCVSS 10.0Exploitation Probability (EPSS)
Elevated risk58th percentile — higher than 58% of all known CVEs
Summary
In FastMCP library prior to version 3.2.0, a vulnerability exists in the RequestDirector class where the _build_url() method does not URL-encode path parameters, allowing path traversal attacks via ../ sequences to access arbitrary backend endpoints. This leads to authenticated SSRF as requests are sent with authorization headers configured in the MCP provider.
Risk Assessment
An attacker can bypass the intended API prefix and access internal backend services, potentially leading to data leakage, privilege escalation, or further attacks on the infrastructure.
Recommendation
Immediately upgrade FastMCP to version 3.2.0 or later, which contains the fix for this vulnerability.
Original NVD description (English source)
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.