Security vendors have warned e-mail users to be as vigilant about PDF attachments as they would for other documents, after seeing a sharp rise in spam embedded within PDF documents.
E-mail security vendor Messagelabs reports that PDF's made up 20 percent of image-based spam messages in July, up 10 percent on the month prior. Image-based spam makes up around 22 percent of total spam, the company said.
The security company believes attackers are using the PDF format due the fact that it more easily bypasses antivirus and anti-spam filters, and that users tend to trust the authenticity of a PDF over other types of documents, even if they don't recognise the sender.
"People have a mindset that the PDF is a locked document," said Andrew Antal, marketing director for MessageLabs. "Anybody can open and make changes to a Word or PowerPoint document sent over e-mail. With a PDF there is a little more assurance that the file in unchangeable, and is thus in a safe state to receive."
Marshal Software CEO Ed McNair says PDF spam is more difficult for an organisation to detect.
In a recent interview with ZDNet Australia, McNair said PDF spam tends to arrive as an attachment in a PDF file. "Once opened it displays the spam message, whether that's a stock trading or an advert for some bogus health product."
"Organisations are finding it very hard to detect PDF spam at the moment, because it doesn't behave in a normal fashion," he said.
Antal said most security software solutions rely on detecting spam by searching for patterns within a message.
"The filtering engines are far smarter when it comes to looking for patterns within Word, PowerPoint on Excel documents than PDFs," he said. "The algorithms are different."
While it is very difficult for an attacker to embed any malware within a PDF file, the spam nonetheless can present a malware risk.
On most PDF spam captured so far, the malware doesn't sit within the PDF and can't be executed by merely opening the PDF, but tends to be hidden in Web links within the document.
A victim would have to not only open the PDF but also click a link within it to risk infection.
"These links are often pointing to Web sites in which malware resides," Antal said.
He said the PDF spam once again shows that organisations need a layered defence to better arm themselves against such threats -- with security software deployed at the gateway, at the client and at the server.
