Date: Mon, 15 Jul 2013 20:21:58 +0200 From: Hector Marco <hecmargi@....es> To: bugtraq@...urityfocus.com, full-disclosure@...ts.grok.org.uk, oss-security@...ts.openwall.com Subject: CVE-2013-4788 - Eglibc PTR MANGLE bug -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 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@....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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with undefined - http://www.enigmail.net/ iQIcBAEBAgAGBQJR5D3CAAoJEI9kAsYMQl6irpMQAJ7vGyBETTHeyURqLmKUgofg YRnK1ia/CszGyNSZVTCF6NIv6JMtaXsF3xvITk0dj68WMtc5DdTSMkw2XQSwzBJv Vlh8QE6KayM+D0esBy6m7+7dLcPyshW4zTKzU6vQwAkxB+PdRKPuEwnVO3hoqtB/ cwiAhk7J2m5sTkKWVz48JJG/f6EjJRZJLaB6J2pzHPijvBCGmTeXLU4+9RnO4i0q CoPcJai5uwDk9yRtj2iwbnHj6PIdSeJj3Sw3UJwZb9vF5gX2FQdSQJTc3yvzc3+7 UMHzuEcScFXWPJpKZGuiHU43sBu9pKvye3MUroEOcG6e4woncABRYRSQzDriN/AJ aUpmvaFtllCA9es286GTBVN7/GGlpLb0PyfdfQW9cVgPqpFZ7Z5GQFMa+pZ/nPRZ gM4aa2YFveQckBJS14yVMz/lyixcVxpEQH0lJbLYO6L9G+0kdaK8knUMR5q9SFYZ GkUzauDkzsGUmDrCvam9mYqc55HOmyQETIfu34SorTOnhD2Seg+BWujbU3BJ2NI1 qIp8SrmX+7V75Jsy9p5/LzkjDXyAoSlwi/RchhtCo5Ih99ZJgjlDrtuR9C+GVBL9 36IemhVfUdM0SFIUJVcCfSMPlrZO/eCCWRnJmTCUBhox9dZ01dUHMNC0h2q/gdxt Bp4l0er3CX70KglD5YIx =gFhS -----END PGP SIGNATURE-----
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