Missing Access Control in the "Free Time" component of several Zyxel UAG, USG, and ZyWall devices allows a remote attacker to generate guest accounts by directly accessing the account generator. This can lead to unauthorised network access or Denial of Service.
https://www.zyxel.com/support/vulnerabilities-related-to-the-Free-Time-feature.shtml
https://n-thumann.de/blog/zyxel-gateways-missing-access-control-in-account-generator-xss/