MCP Server Authentication That Fails Closed: Lessons From The 29 October Advisory

"""Fail-closed authentication for an HTTP MCP server, with a start-up self-test.

CVE-2026-62264 (@sylphx/pdf-reader-mcp, fixed in 3.0.1, published 29 October 2026): the server read
MCP_API_KEY and logged that key authentication was enabled, but never checked the header, and bound to
0.0.0.0 by default. Wrap every route, bind to localhost by default, and refuse to start if an
unauthenticated request gets through. Python 3.14 standard library only (wsgiref).
"""
from __future__ import annotations

import hmac
import os
from collections.abc import Callable, Iterable
from wsgiref.util import setup_testing_defaults

Start = Callable[[str, list[tuple[str, str]]], object]
App = Callable[[dict[str, object], Start], Iterable[bytes]]


def tools(environ: dict[str, object], start: Start) -> Iterable[bytes]:
    start("200 OK", [("Content-Type", "application/json")])
    return [b'{"tools": ["read_pdf"]}']


def require_key(app: App, key: str | None) -> App:
    if not key:
        raise RuntimeError("MCP_API_KEY is not set: refusing to start without authentication")
    def wrapped(environ: dict[str, object], start: Start) -> Iterable[bytes]:
        sent = str(environ.get("HTTP_X_API_KEY", ""))
        if not hmac.compare_digest(sent.encode(), key.encode()):
            start("401 Unauthorized", [("Content-Type", "text/plain")])
            return [b"unauthorised"]
        return app(environ, start)
    return wrapped


def call(app: App, headers: dict[str, str]) -> str:
    env: dict[str, object] = {}
    setup_testing_defaults(env)
    env.update({f"HTTP_{k.upper().replace('-', '_')}": v for k, v in headers.items()})
    status: list[str] = []
    app(env, lambda s, h: status.append(s))
    return status[0]


def self_test(app: App) -> None:
    if not call(app, {}).startswith("401"):
        raise RuntimeError("an unauthenticated request was accepted: refusing to start")


if __name__ == "__main__":
    os.environ["MCP_API_KEY"] = "sample-key-123"
    app = require_key(tools, os.environ.get("MCP_API_KEY"))
    self_test(app)
    print("no key:", call(app, {}), "| wrong key:", call(app, {"X-Api-Key": "nope"}), "| right key:", call(app, {"X-Api-Key": "sample-key-123"}))
    print("bind:", os.environ.get("MCP_BIND", "127.0.0.1"))
    try:
        require_key(tools, None)
    except RuntimeError as e:
        print("unset key:", e)

Python 3.14.4, standard library only. Run once as shown. Uses wsgiref test helpers, no network listener.