Source: https://code.google.com/p/google-security-research/issues/detail?id=478

The Install.framework runner suid root binary does not correctly account for the fact that Distributed Objects
  can be connected to by multiple clients at the same time.

  By connecting two proxy objects to an IFInstallRunner and calling [IFInstallRunner makeReceiptDirAt:asRoot:]
  in the first and passing a custom object as the directory name we can get a callback to our code just after the
  makeReceiptDirAt code has called seteuid(0);setguid(0) to regain privs. Since BSD priviledges are per-process
  this means that our other proxy object will now have euid 0 without having to provide an authorization reference.

  In this second proxy we can then just call runTaskSecurely and get a root shell before returning from the first proxy's callback function
  which will then drop privs.

  build using the provided makefile and run passing the full path to the localhost shell

Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/38136.zip