Spam e-mail using PDF file attachments have all but disappeared with Sophos claiming the rapid drop is because they have failed.
According to Sophos research, levels of PDF spam have dropped from a high of close to 30 percent of all spam earlier this month to virtually zero.
Sophos technology consultant, Graham Cluley, said there can only be one reason for the drop, it isn't working.
"Spammers wouldn't turn away from PDF spam if it was an effective way to fill their pockets with cash and direct consumers to their Web sites, dodgy goods or dodgy investment opportunities," he said.
"The drop in the use of PDFs in spam indicates that the spammers are finding it hard to fool the public into reading their marketing messages distributed in this way."
Sophos experts point to a number of disadvantages for spammers who try and use PDFs in their spam campaigns which may explain its decline.
"PDF spam simply isn't as immediate a way of communicating with your intended audience as an instant glimpse of the marketing message in your e-mail client's preview pane," explained Cluley.
"Furthermore, have you tried opening a PDF file? Adobe Acrobat chugs into action, taking a fair while to load before it can show you the contents of the PDF.
"Consumers pretty quickly learn that it's a waste of time to open every unsolicited PDF they receive, which means the spammer's message doesn't get read, and the cybercriminals don't make any money."
Levels of PDF spam spiked on August 7, 2007 when a single campaign designed to manipulate stock prices accounted for a 30 percent increase in overall junk e-mail levels. Since then, however, PDF spam has shown a sharp decline.
So is this the demise of PDF spam?
"Of course, it's too early to say that this is the last we will see of PDF spam; there could still be more campaigns to come," Cluley said.
Spammers were using PDF files to try and escape detection by e-mail gateway filtering products.
A chart showing the level of PDF spam, is available at: http://www.sophos.com.au/images/sophoslabs-blog/2007/08/pdf-hit-report.jpg
- with Sandra Rossi