BuddyPress 14.4.0 contains a regular expression injection vulnerability in the activity mention resolver that, when username compatibility mode is enabled, allows attackers to manipulate a REGEXP database clause by crafting mention names containing regex metacharacters. Attackers can submit @mentions whose metacharacters pass through esc_sql unescaped and are inserted into an unprepared REGEXP query against the users table, enabling boolean-based inference of usernames and denial of service through catastrophic backtracking.