Didier Stevens

Tuesday 21 October 2008

The Case of the Corrupted Stream Object

Filed under: Malware,PDF,Reverse Engineering — Didier Stevens @ 21:38

A malicious PDF file I analyzed a couple of months ago (the one featured in this video) had a corrupted stream object. It uses a /FlateDecode filter, but I could not find a way to decompress it with the zlib library. Back then, I wrote it off as an error of the malware author.

Lately, I’ve been analyzing some shellcode, and while looking at the shellcode in said malicious PDF, I saw it! The second-stage shellcode, a egghunt shellcode, is searching through process memory for the 8 bytes at the beginning of the corrupted stream object.

The malware author knows that the PDF reader loads the PDF document in memory, so he just overwrote the stream object with his third-stage shellcode. This way, his third-stage shellcode is already in memory, waiting to be found by his second-stage shellcode. And the size of his third-stage shellcode is not limited by the buffer he is overflowing.

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