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AUSCERT External Security Bulletin Redistribution
ESB-2017.0172
K57304814: OpenSSH vulnerability CVE-2016-8858
20 January 2017
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AusCERT Security Bulletin Summary
---------------------------------
Product: F5 LineRate
Publisher: F5 Networks
Operating System: Network Appliance
Impact/Access: Denial of Service -- Remote/Unauthenticated
Resolution: Mitigation
CVE Names: CVE-2016-8858
Reference: ESB-2016.2946
ESB-2016.2583
Original Bulletin:
https://support.f5.com/csp/article/K57304814
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K57304814: OpenSSH vulnerability CVE-2016-8858
Security Advisory
Original Publication Date: Jan 19, 2017
Updated Date: Jan 19, 2017
Applies to (see versions):
Vulnerability Description
** DISPUTED ** The kex_input_kexinit function in kex.c in OpenSSH 6.x and 7.x
through 7.3 allows remote attackers to cause a denial of service (memory
consumption) by sending many duplicate KEXINIT requests. NOTE: a third party
reports that "OpenSSH upstream does not consider this as a security issue.
(CVE-2016-8858)
Impact
An unauthenticated, remote attacker can exploit this, by sending multiple
KEXINIT messages, to consume up to 128 MB per connection. The LineRate system
default MaxSessions option in the sshd_config configuration file is 10, which
limits the impact of this vulnerability to approximately 1 GB of memory
consumed. This vulnerability should only be a concern for very small LineRate
deployments.
Security Issue Status
F5 Product Development has assigned ID LRS-61092 (LineRate) to this
vulnerability.
To determine if your release is known to be vulnerable, the components or
features that are affected by the vulnerability, and for information about
releases or hotfixes that address the vulnerability, refer to the following
table:
Product Versions known to be vulnerable Versions known to be not vulnerable Severity Vulnerable component or feature
BIG-IP LTM None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
11.2.1
BIG-IP AAM None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
BIG-IP AFM None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
BIG-IP Analytics None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
11.2.1
BIG-IP APM None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
11.2.1
BIG-IP ASM None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
11.2.1
BIG-IP DNS None 12.0.0 - 12.1.2 Not vulnerable None
BIG-IP Edge Gateway None 11.2.1 Not vulnerable None
BIG-IP GTM None 11.4.0 - 11.6.1 Not vulnerable None
11.2.1
BIG-IP Link Controller None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
11.2.1
BIG-IP PEM None 12.0.0 - 12.1.2 Not vulnerable None
11.4.0 - 11.6.1
BIG-IP PSM None 11.4.0 - 11.4.1 Not vulnerable None
BIG-IP WebAccelerator None 11.2.1 Not vulnerable None
BIG-IP WebSafe None 12.0.0 - 12.1.2 Not vulnerable None
11.6.0 - 11.6.1
ARX None 6.2.0 - 6.4.0 Not vulnerable None
Enterprise Manager None 3.1.1 Not vulnerable None
BIG-IQ Cloud None 4.0.0 - 4.5.0 Not vulnerable None
BIG-IQ Device None 4.2.0 - 4.5.0 Not vulnerable None
BIG-IQ Security None 4.0.0 - 4.5.0 Not vulnerable None
BIG-IQ ADC None 4.5.0 Not vulnerable None
BIG-IQ Centralized Management None 5.0.0 - 5.1.0 Not vulnerable None
4.6.0
BIG-IQ Cloud and Orchestration None 1.0.0 Not vulnerable None
F5 iWorkflow None 2.0.0 - 2.0.2 Not vulnerable None
LineRate 2.5.0 - 2.6.1 None Low None
Traffix SDC None 5.0.0 - 5.1.0 Not vulnerable None
4.0.0 - 4.4.0
Vulnerability Recommended Actions
If you are running a version listed in the Versions known to be vulnerable
column, you can eliminate this vulnerability by upgrading to a version listed
in the Versions known to be not vulnerable column. If the table lists only an
older version than what you are currently running, or does not list a
non-vulnerable version, then no upgrade candidate currently exists.
Mitigation
To mitigate this vulnerability, you can further reduce the value of
MaxSessions parameter in the sshd_config file. To do so, perform the following
procedure:
Impact of action: This procedure requires you to modify your secure shell
(SSH) configuration, and to restart the SSH service. If you do not update the
configuration syntax correctly, the SSH service may fail to start. When you
restart the SSH service, existing SSH sessions may be terminated. You should
not perform this procedure using a remote SSH session; any mistakes may
prevent further SSH access to the LineRate system. Setting the MaxSessions
value to 0 will prevent all shell, login, and subsystem sessions. You should
test changes during a maintenance window.
Log in to the LineRate command line.
Switch to the Advanced Shell (bash) by typing the following command:
bash
Create a backup of the current sshd_config file by typing the follow command:
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config-K57304814.save
Using a text editor, such as vi or pico, locate the #MaxSessions option within
the /etc/ssh/sshd_config file, uncomment the line by removing the hash tag ( #
) character from the beginning of the line, and then set the desired value.
For example, to limit the number of sessions per connection to 5, locate the
following line:
#MaxSessions 10
Modify the line to match the following example:
MaxSessions 5
Note: You must use the sudo command when opening your text editor, or you will
not have permission to save your changes.
Save your changes and exit the text editor.
Test the syntax of your changes by typing the following two commands:
sudo /usr/linerate/sbin/sshd -t
If there are no errors, this command should return you to the prompt.
echo $?
This echo command returns the status code of the last command you typed. The
result should be 0 (zero).
Important: If either of these commands return errors, repeat step 4 and
confirm that the syntax of the modification is correct. Incorrect
configuration syntax may prevent the SSH service from starting.
Restart the SSH service by typing the following command:
sudo service sshd restart
Note: Existing SSH sessions may be terminated when you restart the SSH
service.
Exit bash by typing the following command:
exit
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