Mainframes – Java – Deserialization

I was asked a week or so ago whether or not I thought z/OS would be susceptible to the types of Java deserialization attacks we’ve seen (a great primer from Fox Glove Security).   Of course!, I replied.  However, I don’t like unsubstantiated claims – so I built this POC:


It uses the basic ysoserial payload generator found on Github.   The file I use to test is from a blog here:

The source is simple:

Simple enough right?    Java on the mainframe is basically Java anywhere.  The only major gotcha (which should come as no surprise to anyone) are with issues of character translation  EBCDIC<->ASCII.   In this case, the ysoserial jarfile I built on x86 and just binary copied it to OMVS and that worked out of the box.

Other times I’ve had to use an a2e / e2a custom decoder – just depends on the implementation.  Currently working to test the JBoss exploits and modify them, if need be, in MSF for z/OS.  More as that unfolds!


While testing this POC first on x86, I kept running into an error like this:

The above mentioned blog helped – Basically Java 1.8u72 (since last December) needs to have the most current version of ysoserial, and use the CommonsCollections5 in order to work (and it does).   Prior versions of Java work just fine with the Release Version (0.04) of ysoserial.

Also, aside from fixes that are library based (like the Adobe Commons Collections one used here), most fixes to this bug have to happen in customized code, often written by organizations.   That makes this vulnerability particularly ugly and potentially difficult to mitigate.