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Re: CVE-2013-4788 - Eglibc PTR MANGLE bug
From: Hector Marco <hecmargi () upv es>
Date: Tue, 16 Jul 2013 00:15:12 +0200


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Due to I signed the mail without care with
the line length the mail is not very readable.

In my personal website you can find a
better format.

http://hmarco.org/bugs/CVE-2013-4788.html


Hector Marco.


On 15/07/13 20:21, Hector Marco wrote:

Hi guys,

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


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