A variant of the Trojan horse attacks known as Storm Worm emerged Monday, targeting people who post blogs and notices to bulletin boards. "The new Storm Worm variant attacks the users' machines when they open an e-mail attachment, click on a malicious e-mail link or visit a malicious site," said Dmitri Alperovitch, principal research scientist at Secure Computing.

When these people later post blogs or bulletin board notices, Storm Worm will insert into each of their postings a link to a malicious Web site.

"We haven't seen the Web channel used before," he said. "In the past, we've seen malicious links distributed to people in a user's address book and made to look like it's an instant message coming from them."