Now PDF Is The Format For Spam Delivery

According to Dmitri Alperovitch, principal research scientist at Secure Computing’s Trusted Source Labs the GIF and JPEG formats have been replaced by PDF files as the new mechanism for delivering their junk mail.

"The makers of e-mail filters have figured out how to recognize image-based spam and have updated their products to stop it. So now [spammers] wrap it up in the PDF," says Alperovitch.

As spam filters have improved, image-based spam has come to a halt. Secure Computing notes that image-based spam has dropped from the 30 percent range to just 10 percent of total e-mail volume in recent months. However, spam in the form of attached PDF files has grown from just one percent in June to five or six percent of e-mail volume in just a month.

For the spammers, it's very easy to generate PDFs since there are many freeware PDF distillers on the market. For anti-spam vendors, it's a much bigger headache. Traditionally, PDF files have not been associated with spam and malware, so very few have been examining PDF files as they pass through the gateway.

The good news after all, according to Alperovitch, is that the PDF files aren't being used to deliver malicious payloads.