if channel.type in ['group', 'dm']:
    if not await Channels.is_user_channel_member(channel.id, user.id, db=db):
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
    # Membership is not authorship — block cross-member edits.
    if user.role != 'admin' and message.user_id != user.id:
        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 await channel_has_access(user.id, channel, permission='write', strict=False, db=db)
    ):
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())

POST   /api/v1/channels/<id>/messages/<victim_msg>/update
       {"content":"wire the funds to account 000",
        "data":{"attacker_injected":true},
        "meta":{"stored_payload":""}}   ->  200
POST   /api/v1/channels/<id>/messages/<victim_msg>/pin                           ->  200
DELETE /api/v1/channels/<id>/messages/<other_msg>/delete                         ->  200, true

else:
    if user.role != 'admin' and not await channel_has_access(
        user.id, channel, permission='write', strict=False, db=db
    ):
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
    # Write access is not authorship — block cross-member edits.
    if user.role != 'admin' and message.user_id != user.id:
        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())

docker run -d -p 3080:8080 -e WEBUI_SECRET_KEY=devkey -e ENABLE_SIGNUP=true \
  -e DEFAULT_USER_ROLE=user --name open-webui ghcr.io/open-webui/open-webui:0.10.2

python3 prep/lab_setup.py                    # administrator, no attack, writes lab.json
python3 exploit_channel_message_tamper.py    # attacker, plus read-only and group-channel controls
python3 exploit_channel_takeover.py --wipe   # attacker, full channel takeover