Look Beyond Executable Files, Botnet Researchers WarnLook Beyond Executable Files, Botnet Researchers Warn

After years of education and awareness training, most users -- and their antivirus applications -- know to be suspicious of email attachments that carry the ".exe" suffix. But users and AV tools are fairly trusting of other types of files -- and botnet operators are beginning to take advantage of that trust.

According to a report being published today by Damballa -- an anti-botnet technology and research firm -- many bot herders are now recruiting their "zombie" participants or levying targeted attacks by hiding malware in real-looking documents that arrive in everyday formats such as Microsoft Office or Adobe Acrobat Reader.

"What we're seeing are a lot more exploits that use everyday document types, such as PDFs or Microsoft Word," says Paul Royal, senior researcher at Damballa, who authored the study. These files are increasingly attached to convincing-looking emails targeted at top executives, claiming to be IRS complaints against the company or quarterly reports, he says.

But end users aren't the only ones who don't recognize these sorts of boobytrapped documents, Royal says. Most antivirus applications -- which generally do a good job of flagging dangerous executable files -- don't recognize malware when it's hidden in conventional document types.

To prove the point, Damballa uploaded 14 new conventional-document attacks to the VirusTotal site, which allows researchers to test new exploits against popular, off-the-shelf security and antivirus products. Of the 32 tools that were tested, only 19 percent could accurately detect the targeted attacks embedded in the standard file types.

"Botnets in general are focusing their efforts more on specific targets, rather than looking to spread themselves to every node, as Storm has done," Royal says. "And botnet operators want their bots to be more reliable and stable, so we see them focusing more on corporate machines."

— Tim Wilson, Site Editor, Dark Reading