Alternately known as Klez.g, Klez.h and Klez.k, depending on the security advisory that's referring to it, the worm has its own e-mail engine to mass mail itself to potential victims, and it also attempts to deactivate some antivirus products. The worm can also spread to shared drives connected to PCs via local area networks or LANs.

While the e-mail message in which the worm gift-wraps itself is relatively standard, its ability to elude most antivirus products has enabled it to spread fairly widely, said Alex Shipp, an antivirus technologist for U.K.-based e-mail service provider MessageLabs.

"The author has changed enough of the bits to get past most virus programs," Shipp said.

While MessageLabs rates the virus as a low threat, Shipp said the rating is updated periodically, and he expects it to reach a high rating when it does update. The company first detected the malicious attachment late Monday and has seen the spread of the worm gradually increase.

Different variants of the Klez worm have generally been among the Top 3 antivirus threats since the first version of the worm was released in January. The Klez.e variant, which appeared last February, was particularly voracious, quickly becoming one of the fastest-spreading worms on the Internet.

Security-software maker Symantec upgraded the latest variant, which it labeled W32.Klez.H, to a threat level of three from a previous rating of two. The company categorizes threats on a scale of one, the lowest threat, to five.

A worm of many subjects
The worm arrives in an e-mail message with one of 120 possible subject lines. There are 18 different standard subject headings, including "let's be friends," "meeting notice," "some questions," and "honey." On top of those, seven other patterns exist, such as "a x game" and "a x patch," where x can be one of 16 different words, including "new," "WinXP," and the name of any of six major antivirus companies.

In many circumstances, the worm doesn't need the victim to open it in order to run. Instead, it takes advantage of a 12-month-old vulnerability in Microsoft Outlook, known as the Automatic Execution of Embedded MIME Type bug, to open itself automatically on unpatched versions of Outlook.

The malicious program will find any network storage available on the infected PC and copy itself to the remote disk drives using a random file name and a .EXE, .PIF, .COM, .BAT, .SCR or .RAR extension. Occasionally, the file name will include a double extension.

The program will also cull e-mail addresses by searching a host of different file types on the infected PC. Using its own mail program, the worm will send itself off to those e-mail addresses. In addition, it will use the addresses to create a fake "From:" field in the e-mail message, disguising the actual source of the e-mail.

Finally, the worm attempts to disable antivirus software by deleting registry keys, stopping running processes and removing virus-definition files.

Clues in the code
The worm also sports a message in its code from the author, who brags that it only took three weeks to create the malicious program.

The author claims the virus originated in Asia and may have bugs because of how fast he created it.

MessageLabs' own data points to China as the source of the first e-mails containing the worm.