Antivirus firms F-Secure and Central Command warned users Tuesday that the Klez.e worm, which can delete files, halt the work of security programs and spread itself when an infected e-mail is opened, is set to trigger Wednesday (Wednesday evening, Thursday morning in Australia). Users are urged to update antivirus definitions and scan their machines as soon as possible.
The Klez worm is much like any other self-propagating, mass mailer worm, in that it harvests e-mail addresses from the Windows address book of infected machines and sends itself to addresses listed there, according to F-Secure. Unlike some other worms, though, Klez also grabs addresses from the chat program ICQ and appears in inboxes with multiple subject lines. Among the subject lines Klez uses are "how are you," "let's be friends," "your password," "some questions" and "congratulations," F-Secure said in its alert. The worm even masquerades as a virus alert, the company said.
The worm is automatically executed when an infected message is opened, according to F-Secure, and infected messages are then sent using an SMTP (Simple Mail Transfer Protocol) engine built in to Klez. Infected messages do not necessarily have an attachment to open -- spreading the worm can occur simply by opening an infected e-mail.
When Klez infects a PC, it installs itself into the registry, infects executable files and kills the tasks launched by security programs running on the PC. Programs targeted include those offered by Symantec Corp., Network Associates Inc., F-Secure Corp., Sophos PLC and Trend Micro Inc. The worm also removes the autostart components of these programs, disabling them, F-Secure said.
The worm has an even more damaging payload, however, that is activated when a certain combination of dates occurs, according to F-Secure. On the sixth day of odd-numbered months (January, March, May, July, September, November) the worm attempts to overwrite all files on the infected PC which have the extensions .txt, .htm, .html, .wab, .doc, .xls, .jpg, .cpp, .c, .pas, .mpg, .mpeg, .bak and .mp3, according to F-Secure. Wednesday, the sixth day of March, an odd numbered month, is such a file-deletion day.
Klez has been around since late 2001, though it has gone through a number of variants.