The infamous Storm worm is back, according to security firm BitDefender, which found the malware in nearly 25 per cent of total detected threats during August.

The Windows metafile rendering exploit, which surfaced at the end of 2005 and was patched in early 2006, came in at second place in the BitDefender top 10 with 21.5 per cent of total detections.

Viorel Canja, head of BitDefender Labs, said that the resurgence is disquieting because it suggests that there are still a lot of unpatched machines around.

The only new entrant into the August top 10 was a sophisticated rootkit which goes by the name of Trojan.Kobcka.A and is probably used to hide a mass-mailer virus. This came in at ninth place with 1.24 per cent of all detections.

BitDefender said that generic Trojan downloader behaviour took third place with 19.06 per cent, while Netsky came in at fourth with 5.89 per cent. The Pandex Trojan topped the top five positions with 2.54 per cent.

"The fact that Trojan.Kobcka racked up more than one per cent of the total malware detections and made it into our top 10 suggests that we may see it, or new variants of it, infecting even more machines over the coming months," said Canja.