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FreeBSD-SA-17:06.openssh Security Advisory
The FreeBSD Project

Topic: OpenSSH Denial of Service vulnerability

Category: contrib
Module: OpenSSH
Announced: 2017-08-10
Affects: All supported versions of FreeBSD.
Corrected: 2017-08-10 06:36:37 UTC (stable/11, 11.1-STABLE)
2017-08-10 06:59:07 UTC (releng/11.1, 11.1-RELEASE-p1)
2017-08-10 06:59:26 UTC (releng/11.0, 11.0-RELEASE-p12)
2017-08-10 06:36:37 UTC (stable/10, 10.3-STABLE)
2017-08-10 06:59:43 UTC (releng/10.3, 10.3-RELEASE-p21)
CVE Name: CVE-2016-6515

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 a built-in password authentication method, which is
enabled with PasswordAuthentication. This option is disabled by
default on FreeBSD.

II. Problem Description

There is no limit on the password length.

III. Impact

A remote attacker may be able to cause an affected SSH server to use
excessive amount of CPU by sending very long passwords, when
PasswordAuthentication is enabled by the system administrator.

IV. Workaround

Disable PasswordAuthentication in /etc/ssh/sshd_config and restart
sshd. This is the default FreeBSD configuration.

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.

Restart SSH service.

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

Restart SSH service.

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-17:06/openssh.patch
# fetch https://security.FreeBSD.org/patches/SA-17:06/openssh.patch.asc
# gpg –verify openssh.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 daemon, or reboot the system.

VI. Correction details

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

Branch/path Revision
– ————————————————————————-
stable/10/ r322341
releng/10.3/ r322344
stable/11/ r322341
releng/11.0/ r322343
releng/11.1/ r322342
– ————————————————————————-

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-2016-6515>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-17:06.openssh.asc>
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