CVE-2026-32871
CriticalCVSS 10.0Exploitation Probability (EPSS)
Elevated risk59th percentile - higher than 59% of all known CVEs
Summary
In FastMCP prior to version 3.2.0, the _build_url() method in the RequestDirector class lacks URL encoding of path parameters. This allows an attacker to inject ../ sequences and perform a path traversal attack, enabling access to arbitrary internal endpoints.
Risk Assessment
An attacker can bypass the intended API prefix and access internal services, leading to authenticated SSRF. This may result in unauthorized access to sensitive data or internal systems.
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.