The proportion of spam messages containing dangerous attachments increased eightfold in recent months, according to security firm Sophos.

During the period from July through September 2008, one in every 416 e-mail messages received carried a dangerous attachment designed to infect PCs with malicious software, according to the company, compared with just one in every 3333e-mail messages be¬tween April and June of the same year.

The spike marked online crooks' return in force to the tactic of attaching mal ware to e7mail after largely ignoring it for a time in favor of planting links to infected Web sites and downloads. Sophos blamed much of the observed increase on several large-scale malware attacks, the worst being the Agent-HNY Trojan horse, which was disguised as

the Penguin Panic arcade game for Apple iPhones. The e-mailed TrOjan horse affected only Windows, however. "Organized criminals are causing havoc for Windows users in the hunt for cold hard cash," says Graham Cluley, senior technology consultant at Sophos. "Too many people are clicking without thinking-exposing themselves to hackers who are hell-bent on gaining access to confidential information and raiding bank accounts."

Noting a rise in the amount of spam sent via social-networking Web sites such as Facebook and Twitter, Sophos says it expects the trend to continue.