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

This script is Copyright (C) 2012 Tenable Network Security, Inc.


Synopsis :

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

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.

- 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.

See also :

http://www.nessus.org/u?61a5078e

Solution :

Update the affected packages.

Risk factor :

Low / CVSS Base Score : 3.5
(CVSS2#AV:N/AC:M/Au:S/C:N/I:N/A:P)

Family: Scientific Linux Local Security Checks

Nessus Plugin ID: 61345 ()

Bugtraq ID:

CVE ID: CVE-2011-5000