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FreeBSD Security Advisory - PAM Policy Parser

FreeBSD Security Advisory - PAM Policy Parser
Posted Jun 4, 2014
Site security.freebsd.org

FreeBSD Security Advisory - The OpenPAM library searches for policy definitions in several locations. While doing so, the absence of a policy file is a soft failure (handled by searching in the next location) while the presence of an invalid file is a hard failure (handled by returning an error to the caller). The policy parser returns the same error code (ENOENT) when a syntactically valid policy references a non-existent module as when the requested policy file does not exist. The search loop regards this as a soft failure and looks for the next similarly-named policy, without discarding the partially-loaded configuration. A similar issue can arise if a policy contains an include directive that refers to a non-existent policy.

tags | advisory
systems | freebsd
advisories | CVE-2014-3879
SHA-256 | c172f1629a0148dc04e340253e113146894af7408276fd65c6e6c6e3a50f19a6

FreeBSD Security Advisory - PAM Policy Parser

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=============================================================================
FreeBSD-SA-14:13.pam Security Advisory
The FreeBSD Project

Topic: Incorrect error handling in PAM policy parser

Category: contrib
Module: pam
Announced: 2014-06-03
Credits: Peter Wemm, Dag-Erling Smørgrav
Affects: FreeBSD 9.2 and later.
Corrected: 2014-06-03 19:02:33 UTC (stable/9, 9.3-BETA1)
2014-06-03 19:02:33 UTC (stable/9, 9.3-BETA1-p1)
2014-06-03 19:03:11 UTC (releng/9.2, 9.2-RELEASE-p7)
2014-06-03 19:02:18 UTC (stable/10, 10.0-STABLE)
2014-06-03 19:02:52 UTC (releng/10.0, 10.0-RELEASE-p4)
CVE Name: CVE-2014-3879

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

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

The OpenPAM library searches for policy definitions in several
locations. While doing so, the absence of a policy file is a soft
failure (handled by searching in the next location) while the presence
of an invalid file is a hard failure (handled by returning an error to
the caller).

The policy parser returns the same error code (ENOENT) when a
syntactically valid policy references a non-existent module as when
the requested policy file does not exist. The search loop regards
this as a soft failure and looks for the next similarly-named policy,
without discarding the partially-loaded configuration.

A similar issue can arise if a policy contains an include directive
that refers to a non-existent policy.

III. Impact

If a module is removed, or the name of a module is misspelled in the
policy file, the PAM library will proceed with a partially loaded
configuration. Depending on the exact circumstances, this may result
in a fail-open scenario where users are allowed to log in without a
password, or with an incorrect password.

In particular, if a policy references a module installed by a package
or port, and that package or port is being reinstalled or upgraded,
there is a brief window of time during which the module is absent and
policies that use it may fail open. This can be especially damaging
to Internet-facing SSH servers, which are regularly subjected to
brute-force scans.

IV. Workaround

If your system uses customized PAM policies, carefully review your
policies to ensure that all module names are spelled correctly.

If your system uses third-party authentication modules, either refrain
from upgrading those modules until you have patched your system, or
shut down the affected services before upgrading.

V. Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) 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 the applicable
FreeBSD release branches.

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

[FreeBSD 9.2]
# fetch http://security.FreeBSD.org/patches/SA-14:13/pam-freebsd9.patch
# fetch http://security.FreeBSD.org/patches/SA-14:13/pam-freebsd9.patch.asc
# gpg --verify pam-freebsd9.patch.asc

[FreeBSD 9.3 and 10.0]
# fetch http://security.FreeBSD.org/patches/SA-14:13/pam-freebsd10.patch
# fetch http://security.FreeBSD.org/patches/SA-14:13/pam-freebsd10.patch.asc
# gpg --verify pam-freebsd10.patch.asc

b) Apply the patch. Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:http://www.FreeBSD.org/handbook/makeworld.html>.

Restart all deamons using the library, or reboot the system.

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

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

VI. Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path Revision
- -------------------------------------------------------------------------
stable/9/ r267015
releng/9.2/ r267018
stable/10/ r267014
releng/10.0/ r267017
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:http://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-3879>

The latest revision of this advisory is available at
<URL:http://security.FreeBSD.org/advisories/FreeBSD-SA-14:13.pam.asc>
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