randhack
========
This is a proof of concept program that can access glibc's randtbl used for
generating pseudo-random values.

This table might exist at an randomized address in the process' memory space.
Calling srand() calls a glibc wrapper that eventually calls _random_r().
_random_r() takes as input a pointer to unsafe_state which is a glibc data
structure that has pointers to randtbl.  _random_r() returns immediately and
control eventually returns to srand's wrapper, then to srand(), and finally the
calling program.

For a 64-bit x86 compiled with particular GCC versions, the rdi register, after
calling srand(,) is never clobbered and thus the pointer to unsafe_state is
available to the user program via the rdi register.

A keen user can read rdi immediately after calling srand() and access
unsafe_state and the randtbl.  Zeroing this table will make rand() and any calls
from rand() via fork() return a deterministic value of '0'  Future calls to
srand() will reset the table.


Files
=====
randhack.c: Example program which demonstrates by calling fork() to show that
clones/forked processes will also share the same (compromised) randtbl in the
shared memory space.

randhack_small.c: Smaller version of the latter, for publication/slides.
                  Does not have a fork() test.


Shouts
======
Hakin9 Magazine <hakin9.org>
2600 Magazine <2600.com>
Ruxcon <ruxcon.org.au>,
Hushcon <hushcon.com>


Contact
=======
mattdavis9@gmail.com (enferex)