Just hours before Adobe is slated to deliver the latest patches for its popular PDF viewer, a security firm announced that by its counting, malicious Reader documents made up 80% of all exploits at the end of 2009.

According to ScanSafe of San Bruno, Calif., vulnerabilities in Adobe's Reader and Acrobat applications were the most frequently targeted of any software during 2009, with hackers' PDF exploits growing throughout the year.

In the first quarter of 2009, malicious PDF files made up 56% of all exploits tracked by ScanSafe. That figure climbed above 60% in the second quarter, over 70% in the third and finished at 80% in the fourth quarter.

"PDF exploits are usually the first ones attempted by attackers," said Mary Landesman, a ScanSafe senior security researcher, referring to the multi-exploit hammering that hackers typically give visitors to malicious Web sites. "Attackers are choosing PDFs for a reason. It's not random. They're establishing a preference for Reader exploits."

Landesman, the author of ScanSafe's just-published annual threat report, said that attackers' preferences for PDF exploits were clearly demonstrated by the data. Exactly why hackers choose Adobe as their prime target is tougher to divine, however.

"Perhaps they are more successful," she said. "Or maybe it's because criminal attackers are human, too. We respond when we see a lot of people going after a particular product.... We all want to go after that product, too. In the attacker arena, they might be thinking, 'Gee, all these reports of Adobe Reader zero-days, maybe I should get in on them too.'"

She also called out the popularity of Reader as a big reason why hackers have pinned a bull's-eye on Adobe. "There's the ubiquitous factor," Landesman said. "PDF use is huge."

Contributing to Adobe's problem is a major increase in vulnerabilities. Landesman's searches of the Common Vulnerabilities and Exposures (CVE) database showed a rapid climb in reported bugs harbored within Adobe's products. In 2009, 107 Abode vulnerabilities were logged into CVE, nearly double the 58 added in 2008 and almost triple the 35 reported in 2006. "There's obviously a lot of activity [by researchers] trying to flush out vulnerabilities from Adobe's software," Landesman said.

"All of these things kind of converge," she added. "I'm not trying to bash Adobe.... Attackers are like electricity, they always follow the path of least resistance. For them, it's 'Tag, you're it,' and Adobe is the one now."

Just as Adobe has done many times itself, Landesman recommended that users disable JavasScript in Reader and Acrobat and steer clear of the Reader browser plug-in.

Later today, Adobe plans to patch several critical vulnerabilities in Reader and Acrobat for Windows, Mac and Linux.

As Landesman intimated, Adobe struggled to keep up with hackers last year. In 2009, Adobe patched four PDF vulnerabilities only after they had already been exploited; 2010 hasn't started out much better, with one PDF zero-day already on the books.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@ix.netcom.com.