Threat Research & Response Blog
The embedded code contains many useless variable assignments and arithmetic operations. After reviewing the block of code, nothing interesting was readily identifiable; most of the code resembled the image shown below:
Image 2 – arithmetic assignments
It is quite different than the obfuscation seen before in other samples. By carefully examining the code, several obfuscations that utilize the reverse process of code optimization are identified. Look at this piece of code:
Image 3 – dead code
The local variable ‘dv’ is assigned a value and has a ‘minus’ operation. But after that, the variable is never accessed in its life scope, which means these two lines are dead code that can be removed without affecting the result of the program. Besides this, fake conditions are added in many places to make the code more confusing:
Image 4 – other meaningless code
The variable ‘dc’ is constant when compared to the constant value ‘7266’; the code block inside the ‘true’ branch of the ‘If’ statement will never be executed, which makes the whole ‘If’ statement useless. After removing all of this dead code, other interesting things begin to show up and all of the strings are encoded in two ways:
Image 5 – decoding algorithm
A function named ‘kop’ is used all in several places to decode the strings using an algorithm. The above example will simply set variable ‘g’ to the value ‘substr’. All external functions are used in the following way so that the strings must decode first before knowing what the function is called.
Is equivalent to this:
Image 8 – de-obfuscated main function
The exploit checks the PDF reader’s version to select exploit shellcode accordingly, then performs a heap spray to try and exploit a vulnerability discussed in CVE-2010-0188. The malformed TIFF data is concatenated as a base64 string and assigned to ‘rawValue’ of XFA field ‘ska’. After the decoding, the TIFF data is shown as following for PDF reader versions between 8.0 to 8.2.1:
Image 9 – TIFF data as viewed in a hex viewer
The shellcode simply downloads and executes additional malware from a remote server. At the time of this writing, the malware was detected as TrojanDownloader:Win32/Epldr.A.
We can see that malware authors are taking obfuscation more seriously to try and evade security software. Although this exploit uses complex obfuscation methods to avoid being detected and analyzed (which makes it more advanced than other exploits) the technique used here is not new.
As always, be safe and use up-to-date security software.
-- Shawn Wang, MMPC