Securing Cisco ASA SSH server


Last year, I wrote a post about securing the Cisco IOS SSH server. It also makes sense to create one for Cisco ASA especially when my old post about enabling SSH on Cisco ASA was back in 2012. That blog post didn’t include the advanced configurations that will improve the security of the Cisco ASA SSH server. With this post, I’d like to share the minimum advanced SSH configurations that network engineers should consider adding to their ASA template.

Enabling Cisco ASA SSH server

Before we can connect to our Cisco ASA via SSH, we need to have a checklist of things we need to configure.

  1. While it’s a good idea to have enable password configured, it is optional for SSH.
  2. You must have at least one user account locally.
  3. Configure ASA’s authentication method. The authentication method can be local, RADIUS, or TACACS+.
  4. Generate RSA or ECDSA key pair.
  5. Configure ACL to allow a specific IP address or range(s).

Setting enable password

My old post covered how to set enable password. It’s the same command on how to set the enable password, but in the newer ASA software, it uses PBKDF2 to encrypt the password compared to the MD5-based hash in older ASA software.

ASA5506(config)# enable password strongpasswordhere
ASA5506(config)# show run | i enable password

Generating RSA keys

As covered in my old post, to enable SSH on the ASA, we’ll need to generate RSA key pair first. Current NIST recommendation is to use 2048-bit or above. In this post, I am going to use 4096-bit key pair.

ASA5506(config)# crypto key generate rsa modulus 4096

SSH Version

Configuring the Cisco ASA SSH server to accept only version 2 is best practice. The reason for this is because SSHv1 has vulnerabilities. That said, make sure to add this to your ASA template.

ASA5506# show ssh
Idle Timeout: 10 minutes
Versions allowed: 1 and 2
<-- Output omitted -->

ASA5506# config t
ASA5506(config)# ssh version 2
ASA5506(config)# show ssh
Idle Timeout: 10 minutes
Version allowed: 2
<-- Output omitted -->

SSH Encryption Algorithms

By default, it seems that the ASA’s encryption algorithm is configured to use the medium settings. Unfortunately, I cannot seem to verify it using the show run all command. However, the combination of show ssh and show ssh ciphers does the trick.

ASA5506# show ssh
Idle Timeout: 10 minutes
Versions allowed: 2
Cipher encryption algorithms enabled:	 aes128-cbc   aes192-cbc   aes256-cbc   aes128-ctr   aes192-ctr   aes256-ctr
<-- Output omitted -->

ASA5506# show ssh ciphers
Available SSH Encryption and Integrity Algorithms
Encryption Algorithms:
	all:	 3des-cbc     aes128-cbc   aes192-cbc   aes256-cbc   aes128-ctr   aes192-ctr   aes256-ctr
	low:	 3des-cbc     aes128-cbc   aes192-cbc   aes256-cbc   aes128-ctr   aes192-ctr   aes256-ctr
	medium:	 aes128-cbc   aes192-cbc   aes256-cbc   aes128-ctr   aes192-ctr   aes256-ctr
	fips:	 aes128-cbc   aes256-cbc
	high:	 aes256-cbc   aes256-ctr
<-- Output omitted -->

The client and server negotiate the encryption algorithm. That said, it is possible that the client would pick a weaker cipher. To avoid that, we’re going to specify the use of a safer cipher. According to this thread, the use of EAX or GCM is preferable when available. If not, the use CTR over CBC mode. By specifying the encryption algorithm, we’re telling ASA to only offer the AES-256-CTR mode to any clients that try to connect to it.

Here’s the verbose output of my SSH to a Cisco ASA using the default SSH cipher encryption.

Mac-mini:~ networkjutsu$ ssh -vvv ASA5506
OpenSSH_7.6p1, LibreSSL 2.6.2
<-- Output omitted -->

debug2: ciphers ctos: aes128-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr
debug2: ciphers stoc: aes128-cbc,aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr
<-- Output omitted -->

Let’s configure the ASA to only use AES256 CTR mode.

ASA5506(config)# ssh cipher encryption custom aes256-ctr
ASA5506(config)# show ssh
Idle Timeout: 10 minutes
Version allowed: 2
Cipher encryption algorithms enabled:	 aes256-ctr
Cipher integrity  algorithms enabled:	 hmac-sha1    hmac-sha1-96

Here’s the verbose output of my SSH connection to a Cisco ASA device using the SSH cipher encryption configuration mentioned above.

Mac-mini:~ networkjutsu$ ssh -vvv ASA5506
OpenSSH_7.6p1, LibreSSL 2.6.2
<-- Output omitted -->

debug2: ciphers ctos: aes256-ctr
debug2: ciphers stoc: aes256-ctr
<-- Output omitted -->

SSH Integrity Algorithm

By default, it seems that the ASA’s integrity algorithm is configured to use the medium settings. Unfortunately, I cannot seem to verify it using the show run all command. However, the combination of show ssh and show ssh ciphers does the trick.

ASA5506# show ssh
Idle Timeout: 10 minutes
Version allowed: 2
Cipher encryption algorithms enabled:	 aes256-ctr
Cipher integrity  algorithms enabled:	 hmac-sha1    hmac-sha1-96
<-- Output omitted -->

ASA5506# show ssh ciphers
Available SSH Encryption and Integrity Algorithms
<-- Output omitted -->
Integrity Algorithms:
	all:	 hmac-sha1    hmac-sha1-96 hmac-md5     hmac-md5-96
	low:	 hmac-sha1    hmac-sha1-96 hmac-md5     hmac-md5-96
	medium:	 hmac-sha1    hmac-sha1-96
	fips:	 hmac-sha1
	high:	 hmac-sha1

The default setting for the ASA SSH integrity algorithm is medium. Which means, it will accept both HMAC-SHA1 and HMAC-SHA1-96. The difference between the two algorithms is the digest length. The HMAC-SHA1-96 is a truncated message digest. From my limited understanding, the HMAC-SHA1-96 is the weakened version of HMAC-SHA1 due to the shortened message digest.

Here’s the verbose output of my SSH to a Cisco ASA using the default SSH integrity algorithm.

Mac-mini:~ networkjutsu$ ssh -vvv ASA5506
OpenSSH_7.6p1, LibreSSL 2.6.2
<-- Output omitted -->

debug2: MACs ctos: hmac-sha1,hmac-sha1-96
debug2: MACs stoc: hmac-sha1,hmac-sha1-96
<-- Output omitted -->

Let’s configure the ASA to only use HMAC-SHA1.

ASA5506(config)# ssh cipher integrity custom aes256-ctr
ASA5506(config)# show ssh
Idle Timeout: 10 minutes
Version allowed: 2
Cipher encryption algorithms enabled:	 aes256-ctr
Cipher integrity  algorithms enabled:	 hmac-sha1

Here’s the verbose output of my SSH connection to a Cisco ASA device using the SSH integrity algorithm configuration mentioned above.

Mac-mini:~ networkjutsu$ ssh -vvv ASA5506
OpenSSH_7.6p1, LibreSSL 2.6.2
<-- Output omitted -->

debug2: MACs ctos: hmac-sha1,hmac-sha1-96
debug2: MACs stoc: hmac-sha1,hmac-sha1-96
<-- Output omitted -->

SSH Key Exchange

The ASA support two Diffie-Hellman key exchange methods and these are DH Group 1 (768-bit) and DH Group 14 (2048-bit). By default, the ASA is set to use Diffie-Hellman Group 1. Unfortunately, this is below what NIST recommends to use in this day and age.

Here’s a Cisco ASA with default SSH key exchange configuration. I issued the no ssh key-exchange to be sure.

ASA5506(config)# no ssh key-exchange
ASA5506(config)# sh run all | i ssh key-exchange
ssh key-exchange group dh-group1-sha1

Here’s the verbose output of my SSH connection to a Cisco ASA using the default SSH key exchange.

Mac-mini:~ networkjutsu$ ssh -vvv ASA5506
OpenSSH_7.6p1, LibreSSL 2.6.2
<-- Output omitted -->
debug1: kex: algorithm: diffie-hellman-group1-sha1
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes256-ctr MAC: hmac-sha1 compression: none
debug1: kex: client->server cipher: aes256-ctr MAC: hmac-sha1 compression: none
<-- Output omitted -->

Let’s configure the ASA to use DH Group 14.

ASA5506(config)# ssh key-exchange group dh-group14-sha1

Here’s the verbose output of my SSH connection to the Cisco ASA after changing the key exchange method.

Mac-mini:~ networkjutsu$ ssh -vvv ASA5506
OpenSSH_7.6p1, LibreSSL 2.6.2
<-- Output omitted -->
debug1: kex: algorithm: diffie-hellman-group14-sha1
debug1: kex: host key algorithm: ssh-rsa
debug1: kex: server->client cipher: aes256-ctr MAC: hmac-sha1 compression: none
debug1: kex: client->server cipher: aes256-ctr MAC: hmac-sha1 compression: none
<-- Output omitted -->

SSH ACL

Restricting remote management to a certain IP address is a best practice. It is also required to add the ACL, or we won’t be able to access the Cisco ASA via SSH. That said, I included the command here.

ssh 192.168.1.0 255.255.255.0 management

Final Words

All of the configurations covered here are what I’d say minimum security standard for all Cisco ASA devices. It is, after all, a network security device, so it is a must to secure it properly. Though this post is just a small part of protecting the management plane and network engineers must incorporate other security configurations.

Want to learn more about ASA?

Cisco ASA: All-in-one Next-Generation Firewall, IPS, and VPN Services (3rd Edition)
Cisco ASA for Accidental Administrators: An Illustrated Step-by-Step ASA Learning and Configuration Guide

Disclosure

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About Andrew Roderos

As an IT professional, I have a strong passion for technology and a desire to learn more about it. Technologies that I am mostly interested in are computer networking, network security, virtualization, and programming. Outside of the information technology world, I enjoy traveling, reading science fiction books and manga, watching movies, and photography.