The following is a bug that we found while we were working around
stack smashing protection techniques.
Title: CVE-2013-4788 - Eglibc PTR MANGLE bug
0.- Description
This bug was discovered in March 2013 while we were developing the RAF SSP
technique. The glibc bug makes it easy to take advantage of common
errors such
as buffer overflows allows in these cases redirect the execution flow and
potentially execute arbitrary code.
1.- Impact
All statically linked applications compiled with glibc and eglibc are affected, independent of the operating system distribution. Note that this problem is not solved by only patching the eglibc, but it is also necessary to recompile all static executables. As far I know there are a lot of routers, embedded systems etc., which use static linked applications. Since the bug is from the beginning of the PTR_MANGLE implementations (years 2005-2006) there are a ton of vulnerable devices.
2.- Vulnerable packages
The bug has been propagated to all the static code compiled with all versions, on all architectures, of glibc from 2.4 (06-Mar-2006) to 2.17 (Current version).
3.- Vulnerability
The vulnerability is caused due to the non initialization to a random value (it is always zero) of the "pointer guard" by the glibc only when generating
static compiled executables. Dynamic executables are not affected. Pointer guard is used to mangle the content of sensible pointers (longjmp, signal handlers, etc.), if the pointer guard value is zero (non-initialized) then it is not effective. An example: Library functions like "setjmp()" or
"longjmp()" use PTR_MANGLE and PTR_DEMANGLE. These macros are used to protect structures like jmp_buf. Basically consist on XOR-ing the pointer value with a random 32/64-bit
value. Since the pointer guard (random value) is 0x0 the attacker can easily
calculate off-line the value of a target address. By overwriting the "env"
structure with the pre-computed address the vulnerability is triggered when
longjmp() is called and the execution flow is redirected to attacker
address.
4.- Exploit
The bug was tested with Debian 7.1 and Ubunu 12.04 LTS and 13.04). I already
created a proof of concept to exploit this vulnerability for both 32 and 64
bits x86 architectures. The proof of concept poc-bug-mangle.c redirect the
execution flow to a function which prompt a shell. This exploit can be
compiled for both i386 and x86_64 architectures. More architectures can be added
easily
by adding the correspondent defines.
Compilation for i386:
gcc poc-bug-mangle.c -o poc-bug-mangle -static
Compilation for x86_64:
gcc poc-bug-mangle.c -o poc-bug-mangle_32 -static -m32
gcc poc-bug-mangle.c -o poc-bug-mangle_64 -static -m64
Execution output:
box () iti upv es:~$ ./poc-bug-mangle
[+] Exploiting ...
[+] hacked !!
$
5.- FIX
Note that the bug is not solved by only patching the eglibc, but it is also
necessary to recompile all static executables. I have created a non official
patch ptr_mangle-eglibc-2.17.patch for the gblic-2.17.
Patching glibc-2.17:
wget http://hmarco.org/bugs/patches/ptr_mangle-eglibc-2.17.patch
cd glibc-2.17
patch -p1 < ../ptr_mangle-eglibc-2.17.patch
6.- Discussion
Although this bug is not exploitable by itself, the truth is that the PTR Mangle encryption is useless. The goal of the protection technique is not achieved. This can be seen as the canary stack is set to 0x0, although is not exploitable by itself is clearly an issue. What about whether the canary has
been set to zero from 2006 to today ? This is what happened with the pointers protected with this mechanism. According to Ulrich_Drepper to use "encryption pointers (instead of canaries) to protect structures like jmp_buf is at least as secure and in addition faster". Following the above and since the protection mechanism is useless from the first implementation, the number of potentially affected systems could be huge.
Patch and exploit source code:
http://hmarco.org/bugs/CVE-2013-4788.html