A giant pump-and-dump scam touting a wireless company's stock boosted on Tuesday the presence of Portable Document Format (PDF) files in spam, increasing the amount of junk e-mail trapped by network sensors by 30 percent over the past 24 hours, antivirus firm Sophos stated.
The jump in spam highlights the increase in usage of certain file types -- including PDF files and Excel files -- to deliver unsolicited bulk e-mail messages. The increased usage of such file types comes at the expense of image files, which accounted for more than half of all spam earlier this year but has fallen to less than 15 percent in July, according to security firm Symantec's latest State of Spam Report.
"Though still steadily declining, what weve come to think of as image spam has not gone away," Symantec researcher Kelly Conley stated in a post on Symantec's blog. "The preferred delivery method of this spam type is now PDF."
Spammers have increasingly moved to formats other than image files in the past six months, responding to companies latest tactic of filtering out messages with certain types of image attachments. The group behind the Storm Worm -- a Trojan horse that creates a bot net to send spam and compromise victims -- has been the first to experiment with new file types, according to researchers.
However, which format is the most popular is still in question: Last month, some companies reported that PDF spam had already begun decreasing, giving way to junk e-mail carrying Excel and ZIP file attachments.
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Posted by: Robert Lemos