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FreeBSD Security Advisory - pam_ssh Improper Access Grant

FreeBSD Security Advisory - pam_ssh Improper Access Grant
Posted Dec 28, 2011
Site security.freebsd.org

FreeBSD Security Advisory - The OpenSSL library call used to decrypt private keys ignores the passphrase argument if the key is not encrypted. Because the pam_ssh module only checks whether the passphrase provided by the user is null, users with unencrypted SSH private keys may successfully authenticate themselves by providing a dummy passphrase. If the pam_ssh module is enabled, attackers may be able to gain access to user accounts which have unencrypted SSH private keys.

tags | advisory
systems | freebsd
MD5 | 21b2c3939bdfec2873616135f939d843

FreeBSD Security Advisory - pam_ssh Improper Access Grant

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=============================================================================
FreeBSD-SA-11:09.pam_ssh Security Advisory
The FreeBSD Project

Topic: pam_ssh improperly grants access when user account has
unencrypted SSH private keys

Category: contrib
Module: pam
Announced: 2011-12-23
Credits: Guy Helmer, Dag-Erling Smorgrav
Affects: All supported versions of FreeBSD.
Corrected: 2011-12-11 20:40:23 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)
2011-12-11 20:38:36 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)
2011-12-11 16:57:27 UTC (RELENG_9, 9.0-STABLE)
2011-12-11 17:32:37 UTC (RELENG_9_0, 9.0-RELEASE)

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 base system includes a module named pam_ssh which, if enabled,
allows users to authenticate themselves by typing in the passphrase of
one of the SSH private keys which are stored in encrypted form in the
their .ssh directory. Authentication is considered successful if at
least one of these keys could be decrypted using the provided
passphrase.

By default, the pam_ssh module rejects SSH private keys with no
passphrase. A "nullok" option exists to allow these keys.

II. Problem Description

The OpenSSL library call used to decrypt private keys ignores the
passphrase argument if the key is not encrypted. Because the pam_ssh
module only checks whether the passphrase provided by the user is
null, users with unencrypted SSH private keys may successfully
authenticate themselves by providing a dummy passphrase.

III. Impact

If the pam_ssh module is enabled, attackers may be able to gain access
to user accounts which have unencrypted SSH private keys.

IV. Workaround

No workaround is available, but systems that do not have the pam_ssh module
enabled are not vulnerable. The pam_ssh module is not enabled in any
of the default policies provided in the base system.

The system administrator can use the following procedure to inspect all
PAM policy files to determine whether the pam_ssh module is enabled.
If the following command produces any output, the system may be
vulnerable:

# egrep -r '^[^#].*\<pam_ssh\>' /etc/pam.* /usr/local/etc/pam.*

The following command will disable the pam_ssh module in all PAM
policies present in the system:

# sed -i '' -e '/^[^#].*pam_ssh/s/^/#/' /etc/pam.conf /etc/pam.d/* \
/usr/local/etc/pam.conf /usr/local/etc/pam.d/*

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 http://security.FreeBSD.org/patches/SA-11:09/pam_ssh.patch
# fetch http://security.FreeBSD.org/patches/SA-11:09/pam_ssh.patch.asc

b) Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch
# cd /usr/src/lib/libpam/modules/pam_ssh
# 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:http://www.FreeBSD.org/handbook/makeworld.html>

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)
utility:

# freebsd-update fetch
# freebsd-update install

VI. Correction details

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

CVS:

Branch Revision
Path
- -------------------------------------------------------------------------
RELENG_7
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.44.2.2
RELENG_7_4
src/UPDATING 1.507.2.36.2.7
src/sys/conf/newvers.sh 1.72.2.18.2.10
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.44.2.1.8.2
RELENG_7_3
src/UPDATING 1.507.2.34.2.11
src/sys/conf/newvers.sh 1.72.2.16.2.13
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.44.2.1.6.2
RELENG_8
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.45.2.3
RELENG_8_2
src/UPDATING 1.632.2.19.2.7
src/sys/conf/newvers.sh 1.83.2.12.2.10
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.45.2.2.4.2
RELENG_8_1
src/UPDATING 1.632.2.14.2.10
src/sys/conf/newvers.sh 1.83.2.10.2.11
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.45.2.2.2.2
RELENG_9
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.47.2.2
RELENG_9_0
src/lib/libpam/modules/pam_ssh/pam_ssh.c 1.47.2.1.2.2
- -------------------------------------------------------------------------

Subversion:

Branch/path Revision
- -------------------------------------------------------------------------
stable/7/ r228421
releng/7.4/ r228843
releng/7.3/ r228843
stable/8/ r228420
releng/8.2/ r228843
releng/8.1/ r228843
stable/9/ r228410
releng/9.0/ r228414
- -------------------------------------------------------------------------

VII. References

The latest revision of this advisory is available at
http://security.FreeBSD.org/advisories/FreeBSD-SA-11:09.pam_ssh.asc
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