Didier Stevens

Tuesday 13 August 2013

A Bit More Than A Signature

Filed under: Encryption,Forensics,Hacking,My Software — Didier Stevens @ 19:07

Soon I’ll release new versions of my Authenticode Tools.

Detecting extra data in the signature field is one of the new features. For example, it will analyze the size specified in the optional header data directory for security, the size specified in the WIN_CERTIFICATE structure and the size specified in the PKCS7 signature itself. These should be the same, taking into account some zero-byte padding.

In case you didn’t know: extra data can be added in the data directory that contains the signature, without invalidating the signature. My Disitool can do this.

With this new version of AnalyzePESig, I found some setup programs that contain extra data after the signature; data that seems to contain installation options for the installer. For example, the Google Chrome installer has this:

20130813-205011

As you can see, the size specified in the optional header data directory for security and the size specified in the WIN_CERTIFICATE structure are both 6272 bytes, but the size of the PKCS7 signature is 6079. So that leaves 181 extra bytes. You can see them here:

20130813-205744

And I found some other installers with extra data (config data or license information) in the signature directory: GotoMyPc, PowerGrep, RegexBuddy.

1 Comment »

  1. […] This year I reported on some installer programs using this padding trick. […]

    Pingback by MS13-098: Fixing Authenticode | Didier Stevens — Wednesday 11 December 2013 @ 23:17


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