Antivirus software decrypts Bagle attachments

The latest security software adds a new trick to its arsenal, decrypting and scanning password-protected attachments

Security firms have started updating their products with more sophisticated techniques aimed at getting inside the encrypted attachments in which the Bagle worm has spread.

Recent versions of the Bagle worm have bypassed corporate gateway security because they are distributed in password-protected Zip files, which are next to impossible for antivirus programs to scan. Emails infected with the Bagle worm, however, contain the password required for opening the Zip file.

On Wednesday, antivirus vendors BitDefender and Kaspersky Labs both launched updates enabling their software to open any encrypted attachments using the password contained in the email text. Once the file is decrypted, it is treated as an executable file and scanned normally.

Eugene Kaspersky, head of antivirus research at Kaspersky Labs, said: "This new technology protects users from a new generation of worms, specifically worms that hide in password-protected Zip files. Five worms using this technique appeared within only four days -- a new trend has been set in the computer underground," he said.

Viorel Canja, head of BitDefender Labs, said in a statement: "We have developed an engine tasked with finding the Zip password in the email text. Most AV products could only offer protection after the archive is extracted; that could be a little too late for inexperienced users," he said.

Network Box, a security appliance vendor that licenses Kaspersky's antivirus software, has updated its gateway product to include complete protection against Bagle, which the company said is a first.

Simon Heron, director of Network Box, told ZDNet UK the product combines Kaspersky's software with Network Box's own technology to deal with the latest Bagle mutations at the network perimeter.

According to Heron, this does mean the gateway is fractionally slower, but by no more than 50ms (milliseconds) per email: "The worst case scenario is we will take 50ms extra to parse an email that has a password-encrypted attachment. We don't think this is a problem," he said.