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Foiled by increasingly accurate corporate spam filters, spammers have dumped pictures for PDFs in their bulk emailings, according to the latest data from security firms.

Image spam, which at the beginning of the year accounted for nearly 60 per cent of all junk email, has plummeted and now accounts for only about 15 per cent of spam.

Taking its place, the number of junk email messages using an attachment in the Portable Document Format (PDF) has steadily climbed since mid-June, accounting for as much as a third of spam.

"It went from zero to - when the spammers started experimenting - 50-50 image spam and PDF spam," said Matt Sergeant, senior anti-spam technologist for email security firm MessageLabs. "Now, it's gone to wholesale PDF spam."

The ebb and flow of different types of spam is an indicator of the arms race between spammers and network defenders. Image spam took off in late 2006, primarily as a way to tout penny stocks and manipulate the volatile over-the-counter markets. Yet, other types of spam, advertising products from fraudulent pharmaceuticals to sexual enhancement devices, soon started using embedded images as well. The growth of image spam peaked earlier this year, making up as much as two-thirds of all spam in January.

Companies have adapted to the attack, however, detecting the unwanted images and blocking them, said MessageLabs' Sergeant.

"The volume of image spam was so great that a number of large businesses took to wholesale blocking of emails coming in with image attachments," he said.

The better filtering has led spammers to change tactics and experiment with PDF files.

While security firms agreed that PDF files started regularly appearing as spam attachment about mid-June, estimates for the volume of PDF spam varied somewhat between companies.

MessageLabs, which filters out virus-laden and spam email messages for its clients, estimated that about 30 per cent of all spam now uses PDF files.

Security firm McAfee had a more modest estimate that 2.6 per cent of all junk email messages carried PDF files. While Symantec, the owner of SecurityFocus, has found the fraction varies between two and seven per cent.

"The spammers are doing the old cat-and-mouse game," said Guy Roberts, senior research manager for anti-spam at McAfee. "Vendors have caught up to spammers and detection is pretty good for image spam, so (the spammers) are changing tactics in order to get their message across."

The growth of spam email messages with PDF attachments have also caused the total bandwidth of spam to grow quickly, because PDF files tend to be much larger than the GIF images that the files are replacing.

From a spammers point of view, the strength of PDF is that many companies require that their email systems allow the documents to be passed to the user, said Menashe Eliezer, director of anti-spam research for security firm CommTouch. Because PDFs are ubiquitous in the business world, such attachments are more likely to reach the users, he said.

"Now, they are using professional looking PDFs, and if it doesn't look like spam, that's even better," Eliezer said.

While moving unwanted advertisements from images to PDFs may make it more likely that the message reaches the intended recipient, whether or not that person opens the attachment is another question, said Doug Bowers, senior director of anti-spam engineering for Symantec.

"We are interested in seeing if this is really effective in getting a spam message, not just delivered, but also read," Bowers said.

In the end, if PDF spam cannot deliver more eyeballs to spammers, the trend may end up being a short-lived phase, he said.

This article originally appeared in Security Focus.

Copyright © 2007, SecurityFocus

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