RHSA-2012:0884 - Security Advisory

Topic

Updated openssh packages that fix one security issue, several bugs, and add
various enhancements are now available for Red Hat Enterprise Linux 6.

The Red Hat Security Response Team has rated this update as having low
security impact. A Common Vulnerability Scoring System (CVSS) base score,
which gives a detailed severity rating, is available from the CVE link in
the References section.

Description

OpenSSH is OpenBSD's Secure Shell (SSH) protocol implementation. These
packages include the core files necessary for the OpenSSH client and
server.

A denial of service flaw was found in the OpenSSH GSSAPI authentication
implementation. A remote, authenticated user could use this flaw to make
the OpenSSH server daemon (sshd) use an excessive amount of memory, leading
to a denial of service. GSSAPI authentication is enabled by default
("GSSAPIAuthentication yes" in "/etc/ssh/sshd_config"). (CVE-2011-5000)

These updated openssh packages also provide fixes for the following bugs:

• SSH X11 forwarding failed if IPv6 was enabled and the parameter

X11UseLocalhost was set to "no". Consequently, users could not set X
forwarding. This update fixes sshd and ssh to correctly bind the port for
the IPv6 protocol. As a result, X11 forwarding now works as expected with
IPv6. (BZ#732955)

• The sshd daemon was killed by the OOM killer when running a stress test.

Consequently, a user could not log in. With this update, the sshd daemon
sets its oom_adj value to -17. As a result, sshd is not chosen by OOM
killer and users are able to log in to solve problems with memory.
(BZ#744236)

• If the SSH server is configured with a banner that contains a backslash

character, then the client will escape it with another "\" character, so it
prints double backslashes. An upstream patch has been applied to correct
the problem and the SSH banner is now correctly displayed. (BZ#809619)

In addition, these updated openssh packages provide the following
enhancements:

• Previously, SSH allowed multiple ways of authentication of which only one

was required for a successful login. SSH can now be set up to require
multiple ways of authentication. For example, logging in to an SSH-enabled
machine requires both a passphrase and a public key to be entered. The
RequiredAuthentications1 and RequiredAuthentications2 options can be
configured in the /etc/ssh/sshd_config file to specify authentications that
are required for a successful login. For example, to set key and password
authentication for SSH version 2, type:

echo "RequiredAuthentications2 publickey,password" >> /etc/ssh/sshd_config

For more information on the aforementioned /etc/ssh/sshd_config options,
refer to the sshd_config man page. (BZ#657378)

• Previously, OpenSSH could use the Advanced Encryption Standard New

Instructions (AES-NI) instruction set only with the AES Cipher-block
chaining (CBC) cipher. This update adds support for Counter (CTR) mode
encryption in OpenSSH so the AES-NI instruction set can now be used
efficiently also with the AES CTR cipher. (BZ#756929)

• Prior to this update, an unprivileged slave sshd process was run as

the sshd_t context during privilege separation (privsep). sshd_t is the
SELinux context used for running the sshd daemon. Given that the
unprivileged slave process is run under the user's UID, it is fitting to
run this process under the user's SELinux context instead of the privileged
sshd_t context. With this update, the unprivileged slave process is now run
as the user's context instead of the sshd_t context in accordance with the
principle of privilege separation. The unprivileged process, which might be
potentially more sensitive to security threats, is now run under the user's
SELinux context. (BZ#798241)

Users are advised to upgrade to these updated openssh packages, which
contain backported patches to resolve these issues and add these
enhancements. After installing this update, the OpenSSH server daemon
(sshd) will be restarted automatically.

Solution

Before applying this update, make sure all previously-released errata
relevant to your system have been applied.

This update is available via the Red Hat Network. Details on how to
use the Red Hat Network to apply this update are available at
https://access.redhat.com/knowledge/articles/11258

Affected Products

• Red Hat Enterprise Linux Server 6 x86_64
• Red Hat Enterprise Linux Server 6 i386
• Red Hat Enterprise Linux Server - Extended Life Cycle Support 6 x86_64
• Red Hat Enterprise Linux Server - Extended Life Cycle Support 6 i386
• Red Hat Enterprise Linux Workstation 6 x86_64
• Red Hat Enterprise Linux Workstation 6 i386
• Red Hat Enterprise Linux Desktop 6 x86_64
• Red Hat Enterprise Linux Desktop 6 i386
• Red Hat Enterprise Linux for IBM z Systems 6 s390x
• Red Hat Enterprise Linux for Power, big endian 6 ppc64
• Red Hat Enterprise Linux for Scientific Computing 6 x86_64
• Red Hat Enterprise Linux Server from RHUI 6 x86_64
• Red Hat Enterprise Linux Server from RHUI 6 i386
• Red Hat Enterprise Linux Server - Extended Life Cycle Support (for IBM z Systems) 6 s390x

Fixes

• BZ - 657378 - [RFE] ability to require password and public-key for login
• BZ - 732955 - X11 forwarding fails, if IPv6 is enabled and X11UseLocalhost=no
• BZ - 797384 - Init scripts tries to read missing file
• BZ - 809938 - CVE-2011-5000 openssh: post-authentication resource exhaustion bug via GSSAPI

CVEs

• CVE-2011-5000

References

• https://access.redhat.com/security/updates/classification/#low