Bagle versions C, D, E, F and G started propagating over the weekend and although the first three are very similar to the original Bagle--being spread through e-mail and infecting PCs of users who open the attachment--Bagle.F and Bagle.G are designed to slip past most enterprise antivirus gateways.
Mikko Hypponen, head of antivirus response at Finnish security company F-Secure, told ZDNet UK that the latest variant of the Bagle family is sent inside an encrypted Zip file attached to an e-mail that contains the password required to access the file. This means that enterprises are unlikely to detect the virus at the perimeter because .zip files are not usually blocked and the encryption means that antivirus scanners will not be able to unzip the file: "This way they get through many gateway scanners that will not be able to unzip the file to scan it."
Graham Cluley, senior technology consultant for antivirus company Sophos, said: "However good an ISP, Web e-mail account or antivirus gateway product may be at scanning e-mail, it will be useless at detecting the worm inside the encrypted Zip file."
David Emm, marketing manager at McAfee Avert, the antivirus company's research arm, agrees: "They are much less likely to block .zip files then they are an .exe, screensaver or .pif file, because it is much more likely that someone puts legitimate data inside a Zip. Also, if it looks like a folder then the user at the receiving end is much more likely to think it is something benign," he said.
Emm commented that he can't remember a worm with this many variants in such a short timeframe: "It's not unusual to see lots of variants, but I can't remember when we have seen so many in such a short amount of time."
Hypponen believes there are so many variants that are similar to each other because they are being modified just enough to keep ahead of the anti virus companies: "C and D are almost identical; F and G are also almost identical. Every time antivirus vendors add a detection, the virus writer/s responds by modifying it a little and releasing the new version," he said:
The Bagle F and G worms are coded to expire on March 25, 2005.
More information on the virues, including prevention and cure, click here.
