(RHSA-2012:0884) Low: openssh security, bug fix, and enhancement update

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.