Just when it appeared tech firms had the upper hand against spam, spammers have unleashed new forms of the meddlesome e-mail to trick filters.

Spam in the form of popular PDF e-mail attachments and electronic greeting cards is confounding e-mail security systems and annoying consumers. The recent Storm e-mail virus and several pump-and-dump stock scams are clogging inboxes and snookering consumers into downloading malicious software. And it could get worse as the holidays approach, anti-spam experts say.

The trend illustrates the shifting nature of spam’s deceptive packaging. As anti-spam vendors come up with solutions, new versions pop up. The most common spam ? which uses images to avoid the detection of spam filters ? is quickly fading because of advances in anti-spam technology.

But spam in PDFs, non-existent in May, now accounts for 8% of unsolicited commercial e-mail. Last week, a PDF promoting a pump-and-dump scam contributed to a 30% increase in overall spam. It was sent from compromised PCs turned into spam-spreading bots, security firm Sophos says.

continued @ USA Today