A did some analysis of some of different possibilities that occur with a single NUL byte overflow into the glibc malloc metadata. Some well-commented little test programs are attached. They all evade the glibc metadata hardening runtime checks to either crash at an attacker-controlled address. Or in one case, the malloc API is confused, in a way that defeats ASLR, to returning multiple pointers to the same actual memory.

At the risk of sounding like a cliche, "the possibilities are endless". I didn't go in to detail on the additional code paths available to the attacker, such as:
- malloc consolidation
- corrupting in to the "top" chunk
- corrupting in to free chunks that are in states other than "unsorted"

I conclude that this condition is extremely exploitable. The malloc metadata hardening is very useful for developers, to highlight corruption early and stop the program closer to the root cause. But if an actual attacker has even a slight amount of control, the hardening can be easily sidestepped.

consolidate_forward.c 786 bytes Download

consolidate_backward.c 727 bytes Download

shrink_free_hole_consolidate_backward.c 1.5 KB Download

shrink_free_hole_alloc_overlap_consolidate_backward.c 2.0 KB Download

geohot set up a wargame for actually exploiting this bug, as a team exercise. It was fun. I've lost the reference to the actual wargame server, but essentially the server allowed you to send messages to malloc() buffers of arbitrary size and content; to free() existing buffers; and to do an off-by-one NUL byte write on an existing buffer.

I've attached my well commented solution to geohot's wargame, heapfunn_solution.c. It is expressed in terms of raw calls to the malloc() / free() API, but could easily be ported to the heapfunn API.

I choose to develop on 64-bit Fedora 20, to show that there's nothing magic about the glibc heap protections on 64-bit. We decided to turn off ASLR for the challenge, to keep matters tractable for a wargame-sized challenge, so to pop your gnome-calculator:

gcc heapfunn_solution.c
setarch x86_64 -R
./a.out

-> gnome-calculator like a boss

heapfunn_solution.c 3.5 KB Download

Attaching my working local Linux privesc exploit. It works on my install; will likely need trivial mods to work on _yours_ :)

pkexploit.c 7.5 KB Download

pwn.c 111 bytes Download

Here is a fixed version portable across installs.

CVE-2014-5119.tar.gz 5.9 KB Download

Blogged on team blog: http://googleprojectzero.blogspot.com/2014/08/the-poisoned-nul-byte-2014-edition.html

This is now fixed.
Sample glibc errata: https://rhn.redhat.com/errata/RHSA-2014-1110.html