Klez

Klez is a mass-mailing Windows computer worm first observed in October 2001. It propagates through email by harvesting recipient addresses from infected users’ address books and sending itself to those contacts in messages with changing subjects, bodies, and attachments. It targeted vulnerable Microsoft Outlook and Outlook Express configurations by abusing an Internet Explorer Trident HTML-rendering flaw that could permit execution through crafted HTML email; other variants relied on deceptive attachments and lures posing as Microsoft patches or Klez-removal tools. Klez spoofed sender addresses using contacts from the victim’s address book, complicating identification of the actual infected sender. It could infect files, replicate through victim environments, and in some variants attach local files to outbound messages, creating a risk of unintended data disclosure. Klez.H was responsible for a major worldwide outbreak in 2002, and Klez remained prevalent in email-borne malware detections for years afterward.