critical

CVE-2026-32871

fastmcp: FastMCP: Authenticated Server-Side Request Forgery via path traversal in OpenAPI path parameters

Published 2 Apr 2026Modified 26 Oct 2026CWE 918

In the Trivy database since at least 24 Oct 2026, 07:11 UTCWhen it arrived before that build is not recorded.

Severity by source

NVD10.0CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

GHSA10.0CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Red Hat8.5CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H

Trivy's own label is CRITICAL. Rated critical by NVD, GHSA. Rated lower by Red Hat (high).

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 packages and fixed versions

Source	Package	Fixed in
pip::GitHub Security Advisory pip	fastmcp	3.2.0
chainguard	mcp-atlassian	0.23.0-r0
echo pip::Echo OSV	fastmcp	2.14.5+echo.1
Red Hat	satellite/foreman-mcp-server-rhel9	no fix listed

References

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