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A security researcher said he has found a new way to exploit Adobe Reader and other PDF-viewing software without using an actual security vulnerability.
The researcher, Didier Stevens, has developed a proof-of-concept PDF file that launches an embedded executable. According to Stevens, PDF viewers like Adobe Reader and Foxit Software’s Foxit Reader do not allow embedded executables to be extracted and run. In his proof-of-concept, however, he was able to execute the embedded executable using a launch action triggered when his PDF file is opened.
Despite his findings, Adobe Systems told eWEEK the protections that exist are enough to defend users.
“The warning message provided in Adobe Reader and Adobe Acrobat includes strong wording advising users to only open and execute the file if it comes from a trusted source,” noted Adobe spokesperson Wiebke Lips.
According to Stevens, the idea of using the launch action functionality first came to his attention about two years ago. Though he did some tests at the time, he noted that a warning was generated when he tried to launch a program, so he did not investigate further.
“A couple of weeks ago, I revisited my research, and noticed that the dialog box had changed, and I found a way to partially control the message displayed by the warning dialog,” said Stevens, an IT security consultant for Contraste Europe. “This motivated me to continue to look for a way to abuse the possibility to pass parameters to cmd.exe, and ultimately execute arbitrary code that I embed in the PDF document.”
He added that he could have chosen to download arbitrary code and execute it as well. In addition, he found that Foxit Reader generates no warning at all, and the action gets executed without user interaction.
Lips said Stevens’ proof-of-concept relies on functionality defined in the PDF specification (ISO PDF 32000-1:2008) and is an example of “powerful functionality relied on by some users that also carries potential risks when used incorrectly.”
“Adobe takes the security of our products and technologies very seriously; we are therefore always listening to and evaluating ways to allow end users and administrators to better manage and configure features like this one to mitigate potential associated risks,” Lips said.
On his blog, Stevens wrote that disabling JavaScript will have no effect on this, and patching Adobe Reader is technically not possible since he is “just being creative with the PDF language specs.”
Ideally, he told eWEEK, Adobe should just disable the feature.
“A quick fix is to remove the code that recognizes the launch keyword,” he added. “But I have no idea how prevalent the use of this launch action is in the world of benign PDF documents.”