CVE-2026-32871: FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability

Severity: critical (CVSS 10.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-02 · updated 2026-10-26
Weakness: CWE-918

Description

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.

Affected products

jlowin fastmcp: before 3.2.0 — fixed in 3.2.0

How to fix

Upgrade to a fixed version: jlowin fastmcp 3.2.0. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-32871 or the next vulnerability affects your stack