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)

Summary

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.

Technical Details

The forwarded terminal identity is a bearer-style authorization claim with no integrity binding, and on the WebSocket path it is additionally injectable because session_id is concatenated into the URL without encoding or delimiter validation.

Impact

A normal authenticated user can make the terminal proxy present another user's identity to the upstream backend coordinator. On backend coordinator-backed (policy_id) servers that scope terminal containers by user_id, this reaches another user's terminal scope; combined with a known active session ID (for example a chat-scoped session ID surfaced through a shared chat), it allows attaching to that user's live PTY. The HTTP-path variant additionally allows identity spoofing at the upstream tier for any deployment whose upstream trusts X-User-Id.

Appendix: Affected code

• backend/open_webui/routers/terminals.py — proxy_terminal sets headers['X-User-Id'] = user.id with no signature.
• backend/open_webui/routers/terminals.py — ws_terminal builds the upstream URL from an unvalidated, unencoded session_id and appends user_id as a query parameter, allowing query injection.

Appendix: Consolidation

Per the Report Handling policy, this consolidates independent reports of the same root cause (the forwarded terminal identity is spoofable / not integrity-bound) into the earliest filing:

• @smoke-wolf (earliest filing) — the X-User-Id HTTP-path identity is forwarded without integrity binding, spoofable where the upstream trusts the header.
• @rexpository — the ws_terminal session_id query-injection vector, proving the forwarded user_id is spoofable through the Open WebUI proxy itself, with no "reach the upstream by other means" precondition.

Appendix: Recommended fix

• Validate and URL-encode session_id before building the upstream URL (urllib.parse.quote(session_id, safe=""); reject ?, #, &, /, %, backslash, control characters). Build the query string with a URL builder so attacker-controlled path content cannot precede it.
• Bind the forwarded identity instead of passing a raw user_id / X-User-Id: emit a short-lived signed claim (for example HS256 over {uid, iat, aud:server_id} with a key shared only with the specific upstream) and verify it upstream.