Klez Worm (2001): One Of The Scariest Computer Viruses Ever Created
Cybercrime Magazine ·
This week in cybersecurity from the editors at Cybercrime Magazine Sausalito, Calif. – Sep. 28, 2026 – Watch the YouTube Video With nearly $20 billion in estimated damages, The Klez Worm infected about 7.2 percent of all computers in 2001, or 7 million PCs. Possibly The post Klez Worm (2001): One Of The Scariest Computer Viruses Ever Created appeared first on Cybercrime Magazine .
Sausalito, Calif. – Sep. 28, 2026
– Watch the YouTube Video
With nearly $20 billion in estimated damages, The Klez Worm infected about 7.2 percent of all computers in 2001, or 7 million PCs.
Possibly originating in China , Klez searched a users’ Microsoft Windows address book and sent fake email messages to all recipients that it found, spoofed recognized senders and, among other things, attempted to deactivate other viruses. Klez was released in several variants. It infected files, copied itself, and spread throughout each victim’s network. It hung around for years, with each version more destructive than the last.
Klez employed a number of random actions that made it hard for many computer users to identify the virus when it arrived in their inboxes. The virus arrived in emails with varying subject lines, or sometimes appeared to be a bounced email or a tool that could purge Klez from an infected system.
Our latest YouTube Short, part of a History of Hacking series produced by Taylor Fox, senior social media manager at Cybercrime Magazine, explains why the Klez mass-mailing worm was one of the scariest computer viruses ever created.
Watch the Video
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The post Klez Worm (2001): One Of The Scariest Computer Viruses Ever Created appeared first on Cybercrime Magazine .