CVE-2026-59224: Open WebUI: Terminal proxy forwards a spoofable, integrity-unbound user identity to the upstream (X-User-Id header and ws_terminal session_id query injection)

The terminal proxy in backend/open_webui/routers/terminals.py forwards the Open WebUI user’s identity to the upstream terminal server / backend coordinator as an authorization claim, with no cryptographic binding to the session that produced it. The forwarded identity is attacker-influenceable on both proxy paths:

1. HTTP path (proxy_terminal) sets headers['X-User-Id'] = user.id. Upstreams that trust X-User-Id as identity receive it unsigned, so an attacker who can reach the upstream by other means (directly, a compromised peer, SSRF) can spoof it.
2. WebSocket path (ws_terminal) is exploitable through Open WebUI itself, with no “other means” required. It interpolates the path parameter session_id directly into the upstream URL and then appends ?user_id=<caller>:

upstream_url = f'{ws_base}/p/{policy_id}/api/terminals/{session_id}'
upstream_url += f'?{urllib.parse.urlencode({"user_id": user.id})}'

session_id is neither validated nor URL-encoded (the HTTP sibling runs _sanitize_proxy_path; this path runs nothing). An encoded ?/& smuggled through session_id survives Open WebUI’s single decode and is re-decoded by the upstream, injecting an attacker-chosen user_id ahead of the appended one. Query parsing binds the first occurrence, so the backend coordinator resolves the spoofed user’s terminal scope.