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.
INFO
Published Date :
2026-04-02T14:52:39.978Z
Last Modified :
2026-04-02T15:59:25.302Z
Source :
GitHub_M
AFFECTED PRODUCTS
The following products are affected by CVE-2026-32871 vulnerability.
| Vendors | Products |
|---|---|
| Prefecthq |
|
REFERENCES
Here, you will find a curated list of external links that provide in-depth information to CVE-2026-32871.
