Open WebUI: Cross-user code-interpreter and tool execution via unvalidated Socket.IO event-caller session_id

Summary

An authenticated low-privilege user can execute arbitrary code-interpreter Python and tools inside another user's authenticated session. The Socket.IO event-caller (get_event_call) delivers execute:python / execute:tool events to a client-supplied session_id after only checking that the session is connected, never that it belongs to the requester. Combined with ydoc:document:join, which exposes the live socket ids of everyone in a shared note's collaboration room to any read-access participant, an attacker can target a victim's session and run attacker-chosen code/tools in the victim's browser context. When the victim is an administrator, that hijacked context reaches the admin-only Functions API, whose source is executed server-side, yielding remote code execution as the server process (root in the default container).

Affected component

• backend/open_webui/socket/main.py — get_event_call() / __event_caller__
• backend/open_webui/main.py — chat-completion metadata (session_id taken from the request body)

Root cause

The event-caller routes to a caller-controlled session id with no ownership check:

# backend/open_webui/socket/main.py — get_event_call()
async def __event_caller__(event_data):
    session_id = request_info['session_id']
    if session_id not in SESSION_POOL:                 # only checks the session is connected
        return {'error': 'Client session disconnected.'}
    return await sio.call('events', {...}, to=session_id, ...)   # delivered to that sid

session_id originates from the request body and is never validated against the authenticated user:

# backend/open_webui/main.py
metadata = {
    'user_id': user.id,                               # server-derived (trustworthy)
    'session_id': form_data.pop('session_id', None),  # client-controlled
    ...
}