This document describes how to disable SSH server CBC mode Ciphers on ASA. On scan vulnerability CVE-2008-5161 it is documented that the use of a block cipher algorithm in Cipher Block Chaining (CBC) mode, makes it easier for remote attackers to recover certain plain text data from an arbitrary block of cipher text in an SSH session via unknown vectors.
Cipher Block Chaining (CBC) is a mode of operation for cipher block, this algorithm uses a block cipher to provide an informational service such as confidentiality or authenticity.
Cisco recommends that you have knowledge of these topics:
The information in this document is based on a Cisco ASA 5506 with OS 9.6.1.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
By default, on the ASA CBC mode is enabled on the ASA which could be a vulnerability for the customers information.
After enhancement CSCum63371, the ability to modify the ASA ssh ciphers was introduced on version 9.1(7), but the release that officially has the commands ssh cipher encryption and ssh cipher integrity is 9.6.1.
In order to disable CBC mode Ciphers on SSH follow this procedure:
Run "sh run all ssh" on the ASA:
ASA(config)# show run all ssh ssh stricthostkeycheck ssh 0.0.0.0 0.0.0.0 outside ssh timeout 60 ssh version 2 ssh cipher encryption medium ssh cipher integrity medium ssh key-exchange group dh-group1-sha1
If you see the command ssh cipher encryption medium this means that the ASA uses medium and high strength ciphers which is setup by default on the ASA.
In order to see the available ssh encryption algorithms in the ASA, run the command show ssh ciphers:
After this is done, run the command show run all ssh, now in the ssh cipher encryption configuration all the algorithms use only CTR mode:
ASA(config)# show run all ssh ssh stricthostkeycheck ssh 0.0.0.0 0.0.0.0 outside ssh timeout 60 ssh version 2 ssh cipher encryption custom "aes128-ctr:aes192-ctr:aes256-ctr" ssh cipher integrity medium ssh key-exchange group dh-group1-sha1
Likewise, the SSH Integrity Algorithms can be modified with the command ssh cipher integrity.