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"It seems repacking malware to attempt avoiding detection, rather than spending time and energy writing new viruses from scratch is a technique that's here to stay," said Sorin Dudea, Head of BitDefender AV Research.

Straightforward packers aren't the only tools used for this purpose. The fourth position on the list is a sophisticated malware loader, dubbed by BitDefender researchers as “Loader.N.” Loader.N runs decrypts and enables two other packed pieces of malware (a Trojan, Kobcka and a Trojan, Downloader.Agent) which transforms the infected machine into a spam relay.

"The loader serves two purposes: the encryption prevents antivirus software from detecting the payload in transit, while the unpacking and loading part ensures that at no point does the payload get written to disk – eliminating another inception point," Dudea explained. "Of course, the exercise is pointless, as we can still detect the decryptor itself."

The rest of the top ten is, yet again, pretty well-populated with mass mailer viruses, with the Cutwail Trojan, another tool turning machines into spam bots, appearing in the 6th position.

BitDefender’s April 2008 Top 10 malware list includes:

1. Packer.Malware.NSAnti.AD 33.71%
2. Win32.Netsky.P@mm 7.48%
3. Win32.Worm.Sohanad.NAW 4.56%
4. Packer.Malware.NSAnti.AG 2.86%
5. Trojan.Loader.N 2.25%
6. Trojan.Dropper.Cutwail.F 2.04%
7. Win32.Netsky.AA@mm 1.98%
8. Win32.NetSky.D@mm 1.98%
9. Packer.Malware.NSAnti.Z 1.87%
10. Win32.Nyxem.E@mm 1.65%
11. OTHERS 39.62%