Are you the Klez monster?

A month after it started spreading, the Klez.h worm isn't slowing down, said antivirus experts on Friday. Moreover, the worm's technique of forging the address of the sender on each infected e-mail message is creating a flood of warnings from gateway antivirus software informing the wrong people that they are infected.

"A lot of traffic is being multiplied by the response mechanisms and refusal mechanisms," said Fred Cohen, security practitioner in residence at the University of New Haven.

In many cases, antivirus software protecting a company's e-mail gateways is sending out a response to each infected e-mail inadvertently sent out by a victim--but that warning is going to the wrong person. "So, in effect, you're getting twice the fun you would normally get," Cohen said.

Apart from magnifying the amount of spam produced by the virus, the incorrect identification of those who are infected is also responsible for hindering efforts to fight the spread of the worm, said Cohen.

Faked addresses
The Klez.h variant, which appeared in mid-April, infects PCs whose users open the attachment to an infected e-mail. Confusing matters, the e-mail will have a random "from" address, selected from various sources on the original victim's hard drive. And it pairs this bogus sender's address with one of more than 120 different subject lines.

When a user opens the attachment, the virus starts up its own e-mail engine and mass mails itself to e-mail addresses found in various files on the PC, using a source address culled from those addresses. Klez.h can also send out a random file from the PC as an attachment, along with the e-mail that carries the worm, potentially passing confidential information.

In some instances, the worm also drops one of several other viruses, including the destructive CIH, and tries to remove any active antivirus software from the system.

Overall, the Klez.h variant has been extremely successful.

"The spread has been really steady," said John Harrington, director of U.S. marketing for e-mail service provider MessageLabs. "We've seen 20,000 again today (Friday), and there's no indication that this is dying down."

While the worm has not spread as quickly as, say, the LoveLetter virus?of which MessageLabs received one copy for every 23 legitimate e-mails during the virus' peak in May 2000--it does make up one out of nearly every 170 e-mails, Harrington said.

In fact, the steady spread--rather than a firestorm of e-mails?may actually be part of the reason for the worm's success, said Harrington. The Klez.h variant did manage to top the charts of computer viruses in April.

"It kind of cruises below the radar screen," Harrington said. "Everyone had heard of LoveLetter. But if you go into a computer shop and ask people if they've heard of Klez, they'll shake their heads."

Hard to track
The Klez variant's ability to spoof the source of infected e-mail makes it nearly impossible to track down the infected users who sent the virus.

"The whole spoofing thing adds a dimension to it that is a little different," said Vincent Gullotto, vice president of Network Associates' antivirus emergency response team. "It's definitely possible that the false addresses are slowing response."

Network Associates still receives more than 50 reports a day of the worm from customers, and some corporate clients are seeing more than 20,000 messages carrying the virus at their e-mail gateways.

The response to Klez--that uninfected users are being told they sent a virus--shows the holes in the system, added Gullotto.

In addition, some out-of-the-office auto-reply mechanisms may be going haywire as a result of an infected user sending an e-mail with a random source and receiver who are both away.

"I am sure there are some auto-reply wars that have been going on," Gullotto said. "There has been a lot of mail that is going around that is caused by this."

Until system administrators disable antivirus notification on the e-mail gateway servers, the confusion will only continue.