Open WebUI: Stored web worker XSS via Pyodide

Title: Same-origin Pyodide code execution allows server-side RCE via a shared chat

Summary

Open WebUI runs client-side Python (Pyodide) in a same-origin web worker. Through Pyodide's JavaScript API (pyodide.http.pyfetch, or the js module which exposes the page's fetch / XMLHttpRequest) executed Python can issue requests on the application origin, and those requests carry the victim's session cookie. A low-privileged user can store such a payload in a chat message, share the chat, and when a victim opens it and clicks Run the payload executes authenticated same-origin requests as the victim. When the victim is an admin (or a user holding workspace.functions / workspace.tools permissions) the payload creates a Function/Tool whose body runs server-side, yielding remote code execution.

Details

Pyodide's js bridge gives Python in the worker the same reach as inline JavaScript on the origin, and the worker is same-origin, so a credentialed request to the app's own API is authenticated as the victim. No separate XSS sink is required: storing the payload in a shared chat and having the victim run it is enough.

 1from pyodide.http import pyfetch
 2import json
 3await pyfetch('/api/v1/functions/create', method='POST', credentials='include',
 4              headers={'Content-Type': 'application/json'},
 5              body=json.dumps({'id': 'x', 'name': 'x', 'meta': {'description': 'x'},
 6                               'content': "import os; os.system('<attacker command>')"}))

Impact

When the victim runs the shared code, an authenticated low-privileged user achieves remote code execution on the server (the created Function/Tool runs server-side Python) if the victim is an admin or holds workspace.functions / workspace.tools permissions. More generally the executed code can issue any authenticated request as the victim. Requires the victim to click Run, and Open WebUI configured to use Pyodide.

Patched

Pyodide now runs in a sandboxed iframe at an opaque origin by default (sandbox="allow-scripts", no allow-same-origin). At an opaque origin pyfetch, fetch and XMLHttpRequest to the app become cross-origin requests that carry no session cookie and are CORS-blocked, and the js bridge operates on the isolated iframe window with no access to the parent's cookie, token, localStorage or DOM. Full Python, JavaScript and external fetch keep working. IDBFS persistence is available only behind ENABLE_PYODIDE_FILE_PERSISTENCE=true, which restores the same-origin worker and re-accepts this risk.

Workaround

Until upgraded, disable Pyodide code execution or set the Code Execution / Code Interpreter engine to a server-side option.

Credits

@gg0h