Four new versions of the Bagle email worm have appeared, and antivirus experts warn that new techniques by the worm's creator could make it harder to stop the new worm variants.
Antivirus companies have issued software updates and alerts about Bagle.Q, R, S and T.
The new versions of the worm, which first appeared in January, do not carry file attachments containing the virus. Instead, they used a months-old Microsoft Windows security hole to break into vulnerable machines, experts said.
"It's really nasty. Just previewing a message in an email client could download the virus to your computer," senior technology consultant at Sophos, Graham Cluley, said.
The security hole used by the worm is known as the Internet Explorer Object Data Remote Execution vulnerability and concerns a problem with the way the Internet Explorer Web browser interprets HTTP (Hypertext Transfer Protocol) data. The vulnerability, MS03-032, was patched by Microsoft in August, 2003. (See: http://www.microsoft.com/technet/security/bulletin/MS03-032.mspx.)
Previous versions of Bagle have shipped off copies of the virus as email file attachments with ZIP, EXE and SCR attachments, among others. Antivirus and antispam products can block the spread of such viruses by scanning incoming email attachments, identifying the virus file by the name, size and other telltale characteristics.
By foregoing file attachments, the Bagle author had made it easier to slip by security products, Cluley said.
Like its predecessors, the new Bagle worms arrive in e-mail messages with faked sender addresses and vague subjects such as "Re: Hello", "Incoming message,", "Site changes", and "Re: Hi."
When opened or previewed on unpatched Windows systems, the Bagle email message first downloads a computer script with a PHP extension from one of a number of predefined Web servers used by the virus author. After it is downloaded, that script runs and downloads, then runs the actual worm file, antivirus company, F-Secure, said.
F-Secure researchers had passed the Internet Protocol (IP) addresses of machines that were hosting the virus file to authorities who were shutting them down, director of antivirus research at F-Secure, Mikko Hyppönen, said.
The new Bagle variants proved that the author was continuing to experiment with new techniques to trick security products, Cluley said.
"There's a continuing evolution with Bagle," he said. "In the beginning there were regular attachments, then they switched to ZIP files, then encrypted ZIP files with passwords, then passwords stored in graphics files, and now this."
The four new variants were closely related and could indicate some tinkering with the worm's code to fix problems, Cluley said.
"There may be some bugs in the code that limited its success," he said.
Antivirus companies said that the Bagle.Q variant, the first in the latest batch, is the most widespread. F-Secure-rated the Bagle.Q a Level 2 threat, indicating large infections within a specific region.
F-Secure has recorded infections in more than 20 countries from Bagle.Q, Hyppönen said.
Sophos had evidence of particularly heavy infections in South Korea, Cluley said.
Antivirus companies posted software updates to detect the new Bagle variants.
Computer users were also advised to apply the Microsoft patch, if they had not already done so, to protect against infection by the new Bagle variants.