Didier Stevens

Monday 6 July 2009

Patching PDF Readers to Support Hidden Embedded Files

Filed under: Hacking,PDF — Didier Stevens @ 20:27

Today, I’m showing you how you can patch your PDF reader (Foxit or Adobe) to handle PDF documents with hidden embedded files. And for Foxit, there’s a bonus: Foxit Reader can also embed files into existing PDF documents.

In my stego PDF trick, I just replace the name /EmbeddedFiles with /Embeddedfiles in the PDF document. As the PDF language is case-sensitive, your PDF reader doesn’t recognize /Embeddedfiles, and hence doesn’t handle the embedded file. PDF readers are designed to skip features of the PDF language they don’t understand (i.e. new features of the PDF language), so that’s why you don’t get an error message from your PDF reader for /Embeddedfiles.

If you search for the string EmbeddedFiles in the binaries of your PDF reader and replace it with Embeddedfiles, it will handle PDF documents with hidden embedded files (but it will stop supporting PDF documents with visible embedded files).

Doing this for Foxit is easy, as there’s only one binary, Foxit Reader.exe. Open it with a hex editor and search for EmbeddedFiles:


Replace it with Embeddedfiles and save it:


That’s it, now you use your patched Foxit Reader to reveal hidden embedded files:


And have you noticed the Add button? Foxit Reader also provides support to add embedded files to existing PDF documents! So you’re not limited to using my Python program to create your own PDF documents.

For Adobe Reader, the trick is the same. Open AcroRd32.dll in a hex editor and do a search and replace (I had to patch 2 instances of of EmbeddedFiles).

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