An e-mail virus that exploits a vulnerability in the Internet Explorer browser and executes when the message is read or previewed. Possibly originating in China, several versions of the Klez worm have been introduced since late 2001. They deposit themselves in the PC's Windows directory with a random file name and set the Registry to launch at startup. Klez searches for e-mail addresses in the infected machine, and, using its own Internet mail transmitter (SMTP engine), sends messages to those addresses with virus attachments and a variety of subject headers and body text. Thus, the same user can keep getting different Klez e-mails from the same source.

In addition, Klez grabs an e-mail address in the infected machine and turns it into the "from" address. If your PC is virus free, but you get e-mails from others saying you are constantly infecting them, those e-mails are coming from some other infected PC that has your e-mail address somewhere in the computer (address book, old message, etc.). In addition, if you keep getting returned, undeliverable messages you never sent, an infected PC is sending e-mails using your from address to recipients whose mail addresses are no longer valid. See e-mail virus.