FreeBSD-SA-11:10.pam: pam_start() does not validate service names

If an application that runs with root privileges allows the user to specify the name of the PAM policy to load, users who are permitted to run that application will be able to execute arbitrary code with root privileges.

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FreeBSD-SA-11:10.pam                                        Security Advisory
                                                          The FreeBSD Project

Topic:          pam_start() does not validate service names

Category:       contrib
Module:         pam
Announced:      2011-12-23
Credits:        Matthias Drochner
Affects:        All supported versions of FreeBSD.
Corrected:      2011-12-13 13:03:11 UTC (RELENG_7, 7.4-STABLE)
                2011-12-23 15:00:37 UTC (RELENG_7_4, 7.4-RELEASE-p5)
                2011-12-23 15:00:37 UTC (RELENG_7_3, 7.3-RELEASE-p9)
                2011-12-13 13:02:52 UTC (RELENG_8, 8.2-STABLE)
                2011-12-23 15:00:37 UTC (RELENG_8_2, 8.2-RELEASE-p5)
                2011-12-23 15:00:37 UTC (RELENG_8_1, 8.1-RELEASE-p7)
                2011-12-13 12:59:39 UTC (RELENG_9, 9.0-STABLE)
                2011-12-13 13:02:31 UTC (RELENG_9_0, 9.0-RELEASE)
CVE Name:       CVE-2011-4122

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL: >.

I.   Background

The PAM (Pluggable Authentication Modules) library provides a flexible
framework for user authentication and session setup / teardown.  It is
used not only in the base system, but also by a large number of
third-party applications.

Various authentication methods (UNIX, LDAP, Kerberos etc.) are
implemented in modules which are loaded and executed according to
predefined, named policies.  These policies are defined in
/etc/pam.conf, /etc/pam.d/<policy name>, /usr/local/etc/pam.conf or
/usr/local/etc/pam.d/<policy name>.

The PAM API is a de facto industry standard which has been implemented
by several parties.  FreeBSD uses the OpenPAM implementation.

II.  Problem Description

Some third-party applications, including KDE's kcheckpass command,
allow the user to specify the name of the policy on the command line.
Since OpenPAM treats the policy name as a path relative to /etc/pam.d
or /usr/local/etc/pam.d, users who are permitted to run such an
application can craft their own policies and cause the application
to load and execute their own modules.

III. Impact

If an application that runs with root privileges allows the user to
specify the name of the PAM policy to load, users who are permitted to
run that application will be able to execute arbitrary code with root

There are no vulnerable applications in the base system.

IV.  Workaround

No workaround is available, but systems without untrusted users are
not vulnerable.

Inspect any third-party setuid / setgid binaries which use the PAM
library and ascertain whether they allow the user to specify the
policy name, then either change the binary's permissions to prevent
its use or remove it altogether.

The following command will output a non-zero number if a dynamically
linked binary uses libpam:

# ldd /usr/local/bin/suspicious_binary | grep -c libpam

The following command will output a non-zero number if a statically
linked binary uses libpam:

# grep -acF "/etc/pam.d/" /usr/local/bin/suspicious_binary

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to 7-STABLE or 8-STABLE, or to
the RELENG_8_2, RELENG_8_1, RELENG_7_4, or RELENG_7_3 security
branch dated after the correction date.

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to FreeBSD 7.4, 7.3,
8.2 and 8.1 systems.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch
# fetch

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/lib/libpam
# make obj && make depend && make && make install

NOTE: On the amd64 platform, the above procedure will not update the
lib32 (i386 compatibility) libraries.  On amd64 systems where the i386
compatibility libraries are used, the operating system should instead
be recompiled as described in
<URL: >

3) To update your vulnerable system via a binary patch:

Systems running 7.4-RELEASE, 7.3-RELEASE, 8.2-RELEASE, or 8.1-RELEASE on
the i386 or amd64 platforms can be updated via the freebsd-update(8)

# freebsd-update fetch
# freebsd-update install

VI.  Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.


Branch                                                           Revision
- -------------------------------------------------------------------------
  src/UPDATING                                             1.507.
  src/UPDATING                                            1.507.
  src/UPDATING                                             1.632.
  src/UPDATING                                            1.632.
- -------------------------------------------------------------------------


Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/7/                                                         r228467
releng/7.4/                                                       r228843
releng/7.3/                                                       r228843
stable/8/                                                         r228466
releng/8.2/                                                       r228843
releng/8.1/                                                       r228843
stable/9/                                                         r228464
releng/9.0/                                                       r228465
- -------------------------------------------------------------------------

VII. References

The latest revision of this advisory is available at
Version: GnuPG v2.0.18 (FreeBSD)

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