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According to Toralv Dirro, a security strategist with McAfee Labs, the percentage of exploitative malware targeting PDF vulnerabilities has skyrocketed. In 2007 and 2008, only 2 percent of all malware that included a vulnerability exploit leveraged an Adobe Reader or Acrobat bug. The number jumped to 17 percent in 2009, and to 28 percent during the first quarter of 2010.
"In the last three years, attackers have found PDF vulnerabilities more and more useful, for a couple of reasons," Dirro said. "First of all, it's increasingly difficult for them to find new vulnerabilities with the operating system and within browsers that they can exploit across the different versions of Windows. And second, Reader is one of the most widely deployed applications that allows files to be accessed or opened within the browser."
Other factors for the jump in PDF exploits, argued Dirro, range from user belief that PDFs are safe to open, or at least safer to open than Microsoft Office documents, to the age of Adobe's code. "Quite a lot of PDF code was written years ago, and attackers are finding new security problems that no one thought of then," Dirro said. "That makes it difficult for Adobe to clean it up."
A recent discovery illustrated Dirro's point. Earlier this month, Belgium researcher Didier Stevens demonstrated how malicious PDFs could use a by-designed feature of the PDF specification to run attack code hidden in the file, and how to modify a warning message that Adobe Reader displays to further trick users into opening the document. Although some of what Stevens revealed has been publicly known for at least eight months, the technique has only been picked up by hackers in the last several weeks.