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5.221. openssh

Updated openssh packages that fix a bug are now available for Red Hat Enterprise Linux 6.
[Updated 12 Nov 2012] This advisory has been updated with an accurate description for BZ#871127 to indicate that the nature of the bug is not architecture-specific. This update does not change the packages in any way.
OpenSSH is OpenBSD's SSH (Secure Shell) protocol implementation. These packages include the core files necessary for both the OpenSSH client and server.

Bug Fix

When SELinux was disabled on the system, no on-disk policy was installed, an user account was used for a connection, and no "~/.ssh" configuration was present in that user's home directory, the ssh client could terminate unexpectedly with a segmentation fault when attempting to connect to another system. A patch has been provided to address this issue and the crashes no longer occur in the described scenario.
All openssh users are advised to upgrade to these updated packages, which fix this bug.
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 for each vulnerability from the CVE link(s) associated with each description below.
OpenSSH is OpenBSD's Secure Shell (SSH) protocol implementation. These packages include the core files necessary for the OpenSSH client and server.

Security Fix

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").

Bug Fixes

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.
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.
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.


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.
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.
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.
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.