FastMCP OpenAPI Provider hat eine SSRF & Path Traversal Vulnerability

The Path Traversal vulnerability in FastMCP allows an attacker to bypass intended access controls and retrieve arbitrary files from the server's file system. By crafting malicious OpenAPI specifications with carefully constructed path parameters containing directory traversal sequences (e.g., ../), an attacker can manipulate the URL used to access backend services. This could lead to the exposure of sensitive configuration files, source code, or other confidential data. The impact is particularly severe because FastMCP is designed to expose APIs, potentially granting access to a wide range of internal resources. Successful exploitation could result in complete system compromise and data exfiltration.

Organizations utilizing FastMCP for managing MCP client API exposure are at risk, particularly those running versions prior to 3.2.0. Environments with less stringent input validation practices or those lacking WAF protection are especially vulnerable. Shared hosting environments where multiple users share the same FastMCP instance could also be affected, as an attacker could potentially exploit the vulnerability to access data belonging to other users.

• python / server:

import os
import urllib.parse

def check_url_encoding(url, base_url):
    parsed_url = urllib.parse.urljoin(base_url, url)
    if '..' in parsed_url:
        print(f"Potential Path Traversal detected: {parsed_url}")

• linux / server:

journalctl -u fastmcp -f | grep "urljoin" # Monitor for URL construction logs

• generic web:

curl -I 'http://your-fastmcp-server/api/v1/users/../sensitive_file.txt' # Attempt directory traversal

1. 2026-03-31Disclosure

   disclosure

1. Reserviert2026-03-16
2. Veröffentlicht2026-03-31
3. Geändert2026-04-10
4. EPSS aktualisiert2026-04-10

The primary mitigation for CVE-2026-32871 is to upgrade FastMCP to version 3.2.0 or later, which contains the fix. If upgrading is not immediately feasible, consider implementing a Web Application Firewall (WAF) rule to block requests containing directory traversal sequences in path parameters. Specifically, look for patterns like ../ or ..%2F in the request URL. Additionally, review and restrict access to the OpenAPI specifications used by FastMCP to prevent unauthorized modification. After upgrading, confirm the fix by attempting to access a restricted file via a crafted path parameter; the request should be rejected.