Executive brief
Open WebUI, a user interface for interacting with Large Language Models (LLMs), contains a flaw in how it handles the very first users who sign up via LDAP or OAuth. If multiple users attempt to log in for the first time simultaneously on a new installation, they could all be granted full administrator privileges. This would allow unauthorized users to access sensitive system configurations, user data, and connected AI backend keys.
Technical details
A Time-of-Check-Time-of-Use (TOCTOU) race condition exists in the LDAP and OAuth registration paths of Open WebUI. The application checks if the user table is empty before inserting a new user; if empty, it assigns the 'admin' role. Because this check and the subsequent insertion are not atomic, multiple concurrent requests on a fresh instance can all satisfy the 'empty table' condition. An attacker can exploit this by sending simultaneous authentication requests to a new deployment to gain unauthorized administrative access. The issue was resolved in version 0.9.0 by implementing an 'insert-first, check-after' pattern to ensure only the absolute first user is promoted to administrator.
Affected products
- Open WebUI open-webui <= 0.8.12
Timeline
- 2026-04-01: patched: First released in v0.9.0
- 2026-05-11: disclosed
- 2026-05-14: advisory