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CVE-2026-32871

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

This page contains facts from public vulnerability sources. Check the original advisory before updating.

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.

CVSS 10.0 · Critical · EPSS 1.4% · Fix available

Published
Updated

Stack: python

Affected products and versions

  • fastmcp

    Versions
    • < 3.2.0: Affected

    No fix published

  • fastmcp PyPI

    Versions
    • 0.1.0
    • 0.2.0
    • 0.3.0
    • 0.3.1
    • 0.3.2
    • 0.3.3
    • 0.3.4
    • 0.3.5
    • 0.4.0
    • 0.4.1
    • 1.0
    • 2.0.0
    • 2.1.0
    • 2.1.1
    • 2.1.2
    • 2.10.0
    • 2.10.1
    • 2.10.2
    • 2.10.3
    • 2.10.4
    • 2.10.5
    • 2.10.6
    • 2.11.0
    • 2.11.1
    • 2.11.2
    • 2.11.3
    • 2.12.0
    • 2.12.0rc1
    • 2.12.1
    • 2.12.2
    • 2.12.3
    • 2.12.4
    • 2.12.5
    • 2.13.0
    • 2.13.0.1
    • 2.13.0.2
    • 2.13.0rc1
    • 2.13.0rc2
    • 2.13.0rc3
    • 2.13.1
    • 2.13.2
    • 2.13.3
    • 2.14.0
    • 2.14.1
    • 2.14.2
    • 2.14.3
    • 2.14.4
    • 2.14.5
    • 2.14.6
    • 2.14.7
    • 2.2.0
    • 2.2.1
    • 2.2.10
    • 2.2.2
    • 2.2.3
    • 2.2.4
    • 2.2.5
    • 2.2.6
    • 2.2.7
    • 2.2.8
    • 2.2.9
    • 2.3.0
    • 2.3.0rc1
    • 2.3.1
    • 2.3.2
    • 2.3.3
    • 2.3.4
    • 2.3.5
    • 2.4.0
    • 2.5.0
    • 2.5.1
    • 2.5.2
    • 2.6.0
    • 2.6.1
    • 2.7.0
    • 2.7.1
    • 2.8.0
    • 2.8.1
    • 2.9.0
    • 2.9.1
    • 2.9.2
    • 3.0.0
    • 3.0.0b1
    • 3.0.0b2
    • 3.0.0rc1
    • 3.0.0rc2
    • 3.0.0rc3
    • 3.0.1
    • 3.0.2
    • 3.1.0
    • 3.1.1
    Affected ranges
    • ECOSYSTEM: < 3.2.0

    Update to 3.2.0

Original sources

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