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FreeBSD-SA-16:14.openssh Security Advisory
The FreeBSD Project

Topic: OpenSSH xauth(1) command injection
Category: contrib
Module: OpenSSH
Announced: 2016-03-16
Credits:
Affects: All supported versions of FreeBSD.
Corrected: 2016-03-12 23:53:20 UTC (stable/10, 10.2-STABLE)
2016-03-14 13:05:13 UTC (releng/10.3, 10.3-RC2)
2016-03-16 22:31:04 UTC (releng/10.2, 10.2-RELEASE-p14)
2016-03-16 22:30:56 UTC (releng/10.1, 10.1-RELEASE-p31)
2016-03-13 23:50:19 UTC (stable/9, 9.3-STABLE)
2016-03-16 22:30:03 UTC (releng/9.3, 9.3-RELEASE-p39)
CVE Name: CVE-2016-3115

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

I. Background

OpenSSH is an implementation of the SSH protocol suite, providing an
encrypted and authenticated transport for a variety of services,
including remote shell access. OpenSSH supports X11 forwarding,
allowing X11 applications on the server to connect to the client’s
display.

When an X11 forwarding session is established, the OpenSSH daemon runs
the xauth tool with information provided by the client to create an
authority file on the server containing information that applications
need in order to connect to the client’s X11 display.

II. Problem Description

Due to insufficient input validation in OpenSSH, a client which has
permission to establish X11 forwarding sessions to a server can
piggyback arbitrary shell commands on the data intended to be passed
to the xauth tool.

III. Impact

An attacker with valid credentials and permission to establish X11
forwarding sessions can bypass other restrictions which may have been
placed on their account, for instance using ForceCommand directives in
the server’s configuration file.

IV. Workaround

Disable X11 forwarding globally by adding the following line to
/etc/ssh/sshd_config, before any Match blocks:

X11Forwarding no

then either restart the OpenSSH daemon or reboot the system.

Consult the sshd(8) and sshd_config(5) manual pages for additional
information on how to enable or disable X11 forwarding on a per-user
or per-key basis.

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,
then either restart the OpenSSH daemon or reboot the system.

2) 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
# service sshd restart

3) 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.

# fetch https://security.FreeBSD.org/patches/SA-16:14/openssh-xauth.patch
# fetch https://security.FreeBSD.org/patches/SA-16:14/openssh-xauth.patch.asc
# gpg –verify openssh-xauth.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:https://www.FreeBSD.org/handbook/makeworld.html>.

d) Either restart the OpenSSH daemon or reboot the system.

VI. Correction details

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

Branch/path Revision
– ————————————————————————-
stable/9/ r296780
releng/9.3/ r296953
stable/10/ r296781
releng/10.1/ r296954
releng/10.2/ r296955
releng/10.3/ r296853
– ————————————————————————-

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:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:http://www.openssh.com/txt/x11fwd.adv>
<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-3115>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-16:14.openssh.asc>
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