Two resurgent email-borne exploits have been hitting users particularly hard over the last week, and researchers say the uptick could get worse.

Stration.DS, a variant of a mass-mailing virus that was first spotted last month, is reproducing at alarming rates, according to researchers. Security vendor Fortinet says it has killed more than 350,000 instances today alone -- more than three times as many as it stopped on Friday; email security vendor Postini confirmed that estimate, saying it has slapped the virus down more than 363,000 times in the last 24 hours.

Separately, Panda Software says it has spotted several variants of the Haxdoor Trojan -- a rootkit exploit that often uses email to steal confidential user information -- over the last seven days. Like Stration, Haxdoor is not new, but appears to be re-emerging in a particularly virulent strain.

Stration, which first appeared in August, is morphing at a breakneck pace. Fortinet has spotted dozens of major variants in the last two months, and the most recent major variant, Stration.DS, has given birth to 495 minor variants since Oct. 18, according to Bryan Lu, an antivirus researcher at Fortinet.

Stration's secret, Lu explains, is it uses previously-infected machines as the launch point for new variants of the virus. With each variant, the virus has a larger base of machines to launch from, and can therefore multiply at a greater rate. Stration.DS also does a better job of concealing itself than previous mass mailer viruses, often hiding as an extension rather than as an obvious executable file, he adds.

"It will probably continue to proliferate at its current rate until more people update their antivirus software and it has a smaller base of machines to launch from," Lu says. "When most people have their antivirus products upgraded, we'll start to see the numbers go down."

Researchers are puzzled by Stration's behavior because it appears to be proliferating without any real purpose. It's possible that it was created by a high school or college student just to see what would happen, Lu says. "Or it may be a means of harvesting a huge number of email addresses as a prelude to a more malicious attack."

The Stration virus is easily blocked, but the rapid proliferation of variants makes it difficult for users to keep their antivirus applications up to date, Lu concedes. Fortinet and other AV vendors are recommending that enterprises take advantage of their "push" services, in which the updated antivirus software is deployed to all nodes two or three times a day as new variants are spotted and remediated.

Users can help, too, by checking the full file information for any file they plan to download, Lu advises. Stration may be hiding as an executable file or as an extension to a seemingly-trustworthy application, he warns.

There is less hard data about Haxdoor, which uses a rootkit to hide from the user and from most antivirus applications that might be running on the PC. Once installed, it hunts for passwords for popular Internet services -- such as eBay, PayPal, or Web Money -- and for popular email clients such as Outlook Express. The attacker can then use the passwords to carry out online fraud or identity theft, Panda says.

Like Stration, Haxdoor is a known threat, but its variants seem to be appearing more rapidly now. The Trojans are capable of making modifications to the PC so that any installed firewall will authorize their malicious processes. This capability makes it possible for the Trojan to send out the stolen passwords and identity information to an attacker, Panda says.

And, like Stration, Haxdoor can be blocked -- if the user has the most current version of an antivirus program, which may change as new variants pop up.

Stration and Haxdoor join Netsky and Mytob as threats that have been attacking email users in wide variants for months at a time. In its monthly threat report issued Friday, Postini said it blocked more than 4 million instances of Netsky alone in September.

"At any given moment in September, Postini was tracking 50,000 computers that were exhibiting signs of malicious behavior," said Scott Petry, founder, chief technical officer, and executive vice president of product development at Postini.

— Tim Wilson, Site Editor, Dark Reading