What you don’t know about PDF can hurt you

Editor’s Note:
Duff Johnson is the President of Netcentric US. This article originally appeared on Appligent.com, and has been reprinted with permission.

Perhaps the single most common myth about PDF files is the idea that they are ‘unchangeable’. WRONG.

It’s the single most common misunderstanding about the most common of file-formats.

In reality, PDF files are easy to change. They are also easy to annotate with comments, encrypt, digitally sign, make interactive, communicate with servers and much much more.

They can also be corrupted with something nasty.

Until recently, these dangers were few and far between. More recently, as IBM’s X-Force Report for 2009 makes clear in gory detail, both PDF and Adobe Systems have ‘taken a beating from attackers over the past one and a half years.’

Let’s review the problem, then discuss some solutions.

Problem 1: Most people think PDF files are inherently secure

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PDF isn’t a closed proprietary format; it’s an open published standard, which makes it possible for some modes of attack to leverage the file-format itself. Until very recently, PDF was targeted far less than the Microsoft Office file formats. The 2009 X-Force Report makes clear that malicious PDF is on the rise, with more vulnerability disclosures about PDF than the various Office formats in 2009.

The most typical attack involving malicious PDF includes ‘trick’ PDF files hosted on servers and emailed as spam or in targeted attacks. These assaults leverage the trust most users place in PDF; users are characteristically less suspicious of .pdf as compared to .doc or .ppt files.

Problem 2: The software

In a more innocent time, certain forms of attack were once considered software features. This is particularly true of so-called ‘XSS’, or cross-site scripting attacks, in which users are spoofed into opening a ‘trick’ PDF file that calls out to a server for various nefarious reasons.

The Acrobat JavaScript API offers an enormous range of options for developers of interactive PDF files including forms, presentations, training materials and more. In fact, the API is so powerful that it’s become an invitation to hackers. Flash content, which may also be embedded in PDFs, presents similar vulnerabilities.

Using PDF viewers other than Adobe’s Reader is no panacea for security concerns. Few 3rd party viewers include Adobe’s comprehensive JavaScript API support and embedded Flash capabilities, but then again, they also don’t have anything like Adobe’s resources for detecting and addressing the threats. Of course, a third-party viewer may have fewer vulnerabilities as a function of their relatively limited features.

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