Adobe considers changes to mitigate PDF attack

Adobe Systems is considering modifying its PDF applications to counter a way to run arbitrary code on Windows computers by embedding it in a malicious PDF file.

Last week, security researcher Didier Stevens detailed a way to run executable code using a different launch command even though PDF applications from Adobe and Foxit don't allow embedded executables to directly run. The attack requires some social engineering.

The particular launch command used by Stevens is defined in the PDF specification (ISO PDF 32000-1:2008) under section 12.6.4.5, wrote Steve Gottwals, a group product manager at Adobe.

"This is a good example of powerful functionality relied upon by some users that also carries potential risks when used incorrectly by others," Gottwals wrote.

Adobe's Reader and Acrobat products do display a warning that only trusted executables should be opened, but Stevens showed how it was possible to modify part of the warning message in order to persuade a user to open the file. The company is considering modifications to the programs.

"We are currently researching the best approach for this functionality in Adobe Reader and Acrobat, which we could conceivably make available during one of the regularly scheduled quarterly product updates," Gottwals wrote.

Older versions of Foxit's Reader did not display a warning message, although the company issued a version 3.2.1 that adds a warning dialog box, according to a company spokesman.

Gottwals wrote that administrators can take steps to mitigate an attack by unchecking a box in the "trust manager" section of "preferences" that allows non-PDF file attachments with external applications to be opened. He also gives instructions for how to modify the registry to prevent users from turning that feature back on.

Although Stevens did not release his entire proof-of-concept code, another security researcher was able to build another kind of attack using part of Stevens' work.

Jeremy Conway, a product manager with NitroSecurity, found a way to spread malicious code across PDF documents on a victim's computer. Conway also modifies the warning message in order to encourage a user to open the malicious PDF. If opened, a malicious payload is added to other PDF files on the computer in a worm-like fashion. The payload could include a Trojan horse program that can log keystrokes and steal passwords.

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