I can’t recall who tweeted it, but an excellent article was published on the analysis of what {W32/Linux/OSX}/Clapzok does. The full article can be downloaded here [Google Drive rendered PDF] and below is the brief intro:

A cross-infector of entirely unrelated platforms is typically implemented as two viruses stuck together, simply because it’s the easiest way to do it. However, if the general mechanics of fi le enumeration and infection are the same across the affected platforms, then a virus can implement an abstraction layer and expose APIs that each of the routines can call to perform the essential functions of find/open/map/unmap/close. This is exactly what {W32/Linux/OSX}/Clapzok does.

The virus begins by calculating the CRC32 of itself. It uses a reverse polynomial (the usual ‘0xEDB88320’) to calculate the hash value. The resulting value is used as the seed for the random number generator in the virus. The virus also relocates the pointers to the abstraction routines, according to the load address of the virus code.

More details can be found here:

• http://www.intego.com/mac-security-blog/clapzok-a-multi-platform-virus/
• http://reverse.put.as/2013/05/31/clapzok-a-reversing-the-os-x-part-of-a-multiplatform-poc-infector/
• http://www.symantec.com/security_response/writeup.jsp?docid=2013-052815-0738-99
• http://www.sophos.com/en-us/threat-center/threat-analyses/viruses-and-spyware/W32~Clapzok-A.aspx