An issue was discovered in the CheckUser extension for MediaWiki before 1.35.12, 1.36.x through 1.39.x before 1.39.5, and 1.40.x before 1.40.1. A user can use a rest.php/checkuser/v0/useragent-clienthints/revision/ URL to store an arbitrary number of rows in cu_useragent_clienthints, leading to a denial of service.