Scientific Linux Security Update : openssh on SL6.x i386/x86_64

Low Nessus Plugin ID 61345


The remote Scientific Linux host is missing one or more security updates.


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.

- 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.

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.

- 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.


Update the affected packages.

See Also

Plugin Details

Severity: Low

ID: 61345

File Name: sl_20120620_openssh_on_SL6_x.nasl

Version: $Revision: 1.1 $

Type: local

Agent: unix

Published: 2012/08/01

Modified: 2012/08/01

Dependencies: 12634

Risk Information

Risk Factor: Low


Base Score: 3.5

Vector: CVSS2#AV:N/AC:M/Au:S/C:N/I:N/A:P

Vulnerability Information

CPE: x-cpe:/o:fermilab:scientific_linux

Required KB Items: Host/local_checks_enabled, Host/cpu, Host/RedHat/release, Host/RedHat/rpm-list

Patch Publication Date: 2012/06/20

Reference Information

CVE: CVE-2011-5000