Adobe has released an emergency update to patch a pair of critical vulnerabilities in its PDF viewing and editing software. The update had been expected: Adobe announced last week that it would issue a rush patch for Adobe Reader and Adobe Acrobat.

The patches came on the same day that a security firm warned that malicious Reader documents made up 80 percent of all exploits in 2009. According to ScanSafe, 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.

As expected, one of the two flaws fixed was related to the cross-domain request vulnerability patched last week in Flash Player, the ubiquitous media player installed on virtually every personal computer. The other bug quashed by today's update was a crash vulnerability that could let attackers install malware on a victimised machine. Adobe ranked both bugs as critical.

Scansafe said tha issues being patched by Adobe were not atypical. In the first quarter of 2009, malicious PDF files made up 56 percent of all exploits tracked by ScanSafe. That figure climbed above 60 percent in the second quarter, over 70 percent in the third and finished at 80 percent 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 websites. "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.

An accompanying security bulletin provides more details of the Reader and Acrobat vulnerabilities patched by Adobe.

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.