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.
- Crypto Key Generate Rsa Command Cisco Router
- Generate A Rsa Crypto Key
- Cisco Command Crypto Key Generate Rsa
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Show command for crypto key. Al, what is the command to see what rsa bit level or if crypto key was configured on the router. Router(config)#crypto key generate rsa.
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.
- While it’s a good idea to have enable password configured, it is optional for SSH.
- You must have at least one user account locally.
- Configure ASA’s authentication method. The authentication method can be local, RADIUS, or TACACS+.
- Generate RSA key pair.
- Configure ACL to allow a specific IP address or range(s).
Setting enable password
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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.
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.
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.
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.
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.
Generate A Rsa Crypto Key
Here’s the verbose output of my SSH to a Cisco ASA using the default SSH cipher encryption.
Let’s configure the ASA to only use AES256 CTR mode.
Here’s the verbose output of my SSH connection to a Cisco ASA device using the SSH cipher encryption configuration mentioned above.
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.
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.
Let’s configure the ASA to only use HMAC-SHA1.
I'am trying to setup an IPsec IKEv2 tunnel where the local authentication mechanism is based on RSA. Crypto key generate rsa command not working.
Here’s the verbose output of my SSH connection to a Cisco ASA device using the SSH integrity algorithm configuration mentioned above.
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.
Here’s the verbose output of my SSH connection to a Cisco ASA using the default SSH key exchange.
Let’s configure the ASA to use DH Group 14.
Here’s the verbose output of my SSH connection to the Cisco ASA after changing the key exchange method.
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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.
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.
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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
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