In a change of tactics, the attackers have hidden the spam content within a PDF file instead of attaching an image file to plug the stock, according to a security advisory on the McAfee website.
The spammers are sending the PDF files with randomly generated subject lines, sender names and a blank message body.
The stock spam is believed to have been sent from Stration infected computers, as this attack is similar to the W32/Stration worm mass-mailing, which contained a number of PDF files, Nick Kelly, sustaining engineer at McAfee said.
"Spammers are struggling to find ways to fool spam filters and get their messages into people's inboxes," said Bradley Anstis, director of product management at Marshal.
“But, spammers believe many anti-spam solutions largely ignore PDF files, so they use them in an attempt to add credibility and legitimacy to their messages. We expect to see a lot more of PDF spam. This recent case is just the beginning."
Hackers launch PDF spam campaign
By Fiona Raisbeck on Jun 29, 2007 9:55AM