Executive brief
Open WebUI is a web-based platform for managing chat, automations, and calendar events. An authenticated regular user can disable calendar alerts for all users on the instance by storing a text string instead of a number in a calendar event's alert configuration. The impact is limited to calendar alert functionality—notifications fail to send for roughly an hour per malicious event—but affects all users simultaneously and can be sustained indefinitely.
Technical details
The vulnerability is a type confusion and missing input validation issue (CWE-1287, CWE-754) in Open WebUI's calendar event model. Calendar events carry a free-form `meta` dictionary stored without type enforcement. The scheduler's alert pass reads the alert offset from this dictionary and performs numeric comparison without type-checking, assuming prior validation. When a user stores a string (e.g., `"5"`) instead of a numeric value, the scheduler raises a type error mid-pass, aborting the entire instance-wide alert check. This affects all users regardless of permissions. The attack requires: (1) calendar enabled, (2) attacker with calendar feature permission (default for all users), and (3) event start within the one-hour lookahead window. No admin, shared calendars, or recurrence rules are required. Fixed in version 0.11.1 (PR #28790) by treating non-numeric alert values as unset and falling back to defaults; events already holding bad values revert automatically.
Affected products
- Open WebUI Contributors Open WebUI >= 0.9.0, < 0.11.1
Timeline
- 2026-09-04: disclosed
- 2026-09-10: advisory
- 2026-09-04: patched: Fix released in version 0.11.1, PR #28790