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FreeBSD Security Advisory - sshd Denial Of Service

FreeBSD Security Advisory - sshd Denial Of Service
Posted Nov 5, 2014
Site security.freebsd.org

FreeBSD Security Advisory - Although OpenSSH is not multithreaded, when OpenSSH is compiled with Kerberos support, the Heimdal libraries bring in the POSIX thread library as a dependency. Due to incorrect library ordering while linking sshd(8), symbols in the C library which are shadowed by the POSIX thread library may not be resolved correctly at run time. Note that this problem is specific to the FreeBSD build system and does not affect other operating systems or the version of OpenSSH available from the FreeBSD ports tree. An incorrectly linked sshd(8) child process may deadlock while handling an incoming connection. The connection may then time out or be interrupted by the client, leaving the deadlocked sshd(8) child process behind. Eventually, the sshd(8) parent process stops accepting new connections. An attacker may take advantage of this by repeatedly connecting and then dropping the connection after having begun, but not completed, the authentication process.

tags | advisory
systems | freebsd, osx
advisories | CVE-2014-8475
MD5 | 9c604890b80de664fad4262cb5aeb66b

FreeBSD Security Advisory - sshd Denial Of Service

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=============================================================================
FreeBSD-SA-14:24.sshd Security Advisory
The FreeBSD Project

Topic: Denial of service attack against sshd(8)

Category: contrib
Module: openssh
Announced: 2014-11-04
Credits:
Affects: FreeBSD 9.1, 9.2 and 10.0.
Corrected: 2014-05-04 07:28:26 UTC (stable/10, 10.0-STABLE)
2014-11-04 23:31:17 UTC (releng/10.0, 10.0-RELEASE-p12)
2014-05-04 07:57:20 UTC (stable/9, 9.2-STABLE)
2014-11-04 23:33:17 UTC (releng/9.2, 9.2-RELEASE-p15)
2014-11-04 23:32:45 UTC (releng/9.1, 9.1-RELEASE-p22)
CVE Name: CVE-2014-8475

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

OpenSSH is an implementation of the SSH protocol suite, providing an
encrypted and authenticated transport for a variety of services,
including remote shell access. The sshd(8) daemon is the server side
of OpenSSH.

Heimdal is an implementation of Kerberos 5, which provides
authentication and single sign-on capability for many network
services, including OpenSSH.

II. Problem Description

Although OpenSSH is not multithreaded, when OpenSSH is compiled with
Kerberos support, the Heimdal libraries bring in the POSIX thread
library as a dependency. Due to incorrect library ordering while
linking sshd(8), symbols in the C library which are shadowed by the
POSIX thread library may not be resolved correctly at run time.

Note that this problem is specific to the FreeBSD build system and
does not affect other operating systems or the version of OpenSSH
available from the FreeBSD ports tree.

III. Impact

An incorrectly linked sshd(8) child process may deadlock while
handling an incoming connection. The connection may then time out or
be interrupted by the client, leaving the deadlocked sshd(8) child
process behind. Eventually, the sshd(8) parent process stops
accepting new connections.

An attacker may take advantage of this by repeatedly connecting and
then dropping the connection after having begun, but not completed,
the authentication process.

IV. Workaround

Possible workarounds include rebuilding sshd with Kerberos support
disabled or installing the security/openssh-portable package from the
FreeBSD ports tree or an official package repository.

Systems that do not run an OpenSSH server are not affected.

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 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

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 http://security.FreeBSD.org/patches/SA-14:24/sshd.patch
# fetch http://security.FreeBSD.org/patches/SA-14:24/sshd.patch.asc
# gpg --verify sshd.patch.asc

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

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

c) Recompile sshd. Execute the following commands as root:

# cd /usr/src/secure/usr.sbin/sshd
# make && make install

4) Restart the affected service

To restart the affected service after updating the system, either
reboot the system or execute the following command as root:

# service sshd restart

VI. Correction details

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

Branch/path Revision
- -------------------------------------------------------------------------
stable/9/ r265314
releng/9.1/ r274112
releng/9.2/ r274113
stable/10/ r265313
releng/10.0/ r274110
- -------------------------------------------------------------------------

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

<other info on vulnerability>

<URL:http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-8475>

The latest revision of this advisory is available at
<URL:http://security.FreeBSD.org/advisories/FreeBSD-SA-14:24.sshd.asc>
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