Klez is a computer worm that propagates via e-mail. It first appeared in October 2001. A number of variants of the worm exist. Klez infects Microsoft Windows systems, exploiting a vulnerability in Internet Explorer's Trident layout engine, used by both Microsoft Outlook and Outlook Express to render HTML mail. Once the worm is executed, either automatically by the buggy HTML engine or manually by a user, it searches for addresses to send itself to. When it sends itself out, it may attach a file from the infected machine, leading to possible privacy breaches.