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Posted on 28 July 2015
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FreeBSD-SA-15:16.openssh Security Advisory
The FreeBSD Project
Topic: OpenSSH multiple vulnerabilities
Category: contrib
Module: openssh
Announced: 2015-07-28
Affects: All supported versions of FreeBSD.
Corrected: 2015-07-28 19:58:44 UTC (stable/10, 10.2-PRERELEASE)
2015-07-28 19:58:44 UTC (stable/10, 10.2-BETA2-p2)
2015-07-28 19:59:04 UTC (releng/10.2, 10.2-RC1-p1)
2015-07-28 19:59:11 UTC (releng/10.1, 10.1-RELEASE-p16)
2015-07-28 19:58:54 UTC (stable/9, 9.3-STABLE)
2015-07-28 19:59:22 UTC (releng/9.3, 9.3-RELEASE-p21)
2015-07-28 19:58:54 UTC (stable/8, 8.4-STABLE)
2015-07-28 19:59:22 UTC (releng/8.4, 8.4-RELEASE-p35)
CVE Name: CVE-2014-2653, CVE-2015-5600
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.
The security of the SSH connection relies on the server authenticating
itself to the client as well as the user authenticating itself to the
server. SSH servers uses host keys to verify their identity.
RFC 4255 has defined a method of verifying SSH host keys using Domain
Name System Security (DNSSEC), by publishing the key fingerprint using
DNS with "SSHFP" resource record. RFC 6187 has defined methods to use
a signature by a trusted certification authority to bind a given public
key to a given digital identity with X.509v3 certificates.
The PAM (Pluggable Authentication Modules) library provides a flexible
framework for user authentication and session setup / teardown.
OpenSSH uses PAM for password authentication by default.
II. Problem Description
OpenSSH clients does not correctly verify DNS SSHFP records when a server
offers a certificate. [CVE-2014-2653]
OpenSSH servers which are configured to allow password authentication
using PAM (default) would allow many password attempts.
III. Impact
A malicious server may be able to force a connecting client to skip DNS
SSHFP record check and require the user to perform manual host verification
of the host key fingerprint. This could allow man-in-the-middle attack
if the user does not carefully check the fingerprint. [CVE-2014-2653]
A remote attacker may effectively bypass MaxAuthTries settings, which would
enable them to brute force passwords. [CVE-2015-5600]
IV. Workaround
Systems that do not use OpenSSH are not affected.
There is no workaround for CVE-2014-2653, but the problem only affects
networks where DNSsec and SSHFP is properly configured. Users who uses
SSH should always check server host key fingerprints carefully when
prompted.
System administrators can set:
UsePAM no
In their /etc/ssh/sshd_config and restart sshd service to workaround the
problem described as CVE-2015-5600 at expense of losing features provided
by the PAM framework.
We recommend system administrators to disable password based authentication
completely, and use key based authentication exclusively in their SSH server
configuration, when possible. This would eliminate the possibility of being
ever exposed to password brute force attack.
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.
SSH service has to be restarted after the update. A reboot is recommended
but not required.
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
SSH service has to be restarted after the update. A reboot is recommended
but not required.
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.
[FreeBSD 9.3, 10.1, 10.2]
# fetch https://security.FreeBSD.org/patches/SA-15:16/openssh.patch
# fetch https://security.FreeBSD.org/patches/SA-15:16/openssh.patch.asc
# gpg --verify openssh.patch.asc
[FreeBSD 8.4]
# fetch https://security.FreeBSD.org/patches/SA-15:16/openssh-8.patch
# fetch https://security.FreeBSD.org/patches/SA-15:16/openssh-8.patch.asc
# gpg --verify openssh-8.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>.
Restart the SSH service, or reboot the system.
VI. Correction details
The following list contains the correction revision numbers for each
affected branch.
Branch/path Revision
- -------------------------------------------------------------------------
stable/8/ r285977
releng/8.4/ r285980
stable/9/ r285977
releng/9.3/ r285980
stable/10/ r285976
releng/10.1/ r285979
releng/10.2/ r285978
- -------------------------------------------------------------------------
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:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-2653>
<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-5600>
The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-15:16.openssh.asc>
