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FreeBSD Security Advisory - OpenSSH Record Check

FreeBSD Security Advisory - OpenSSH Record Check
Posted Jul 28, 2015
Site security.freebsd.org

FreeBSD Security Advisory - OpenSSH clients does not correctly verify DNS SSHFP records when a server offers a certificate. OpenSSH servers which are configured to allow password authentication using PAM (default) would allow many password attempts.

tags | advisory
systems | freebsd
advisories | CVE-2014-2653, CVE-2015-5600
SHA-256 | 5a62702946b5a02f2793adee927547243f7fc23df83ae91a601fe9c2411fbd69

FreeBSD Security Advisory - OpenSSH Record Check

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Hash: SHA512

=============================================================================
FreeBSD-SA-15:16.openssh Security Advisory
The FreeBSD Project

Topic: OpenSSH multiple vulnerabilities

Category: contrib
Module: openssh
Announced: 2015-07-28, revised on 2015-07-30
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)
2015-07-30 10:09:07 UTC (stable/8, 8.4-STABLE)
2015-07-30 10:09:31 UTC (releng/8.4, 8.4-RELEASE-p36)
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/>.

0. Revision history

v1.0 2015-02-25 Initial release.
v1.1 2015-07-30 Revised patch for FreeBSD 8.x to address regression when
keyboard interactive authentication is used.

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

# fetch https://security.FreeBSD.org/patches/SA-15:16/openssh-8-errata.patc
# fetch https://security.FreeBSD.org/patches/SA-15:16/openssh-8-errata.patch.asc
# gpg --verify openssh-8-errata.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/ r286067
releng/8.4/ r286068
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>
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