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41Track*

FastMCP OpenAPI Provider has an SSRF & Path Traversal Vulnerability

PrefectHQ fastmcp · CVE-2026-32871 GHSA-VV7Q-7JX5-F767
0 news outlets0 CERTs and agencies0 research labs

Assessment: Track*

SSVC decision points: exploitation poc, automatable yes, technical impact total, deployment low.

What is it?

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.

Evidence

No coverage recorded yet.

Exploitation

  • Exploit code published (not verified by us)
  • Exploited in the wild (confirmed by a source)
  • Listed as known exploited (CISA KEV)
CVSS
10.0
EPSS
–
First seen
31 Mar 2026
Status
corroborated

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