Disable weak ciphers on NSX Manager - scanning reports show "SSH Weak Message Authentication Code Algorithms"

Products

• VMware NSX

Issue/Introduction

Symptoms:

• Security scanner has determined that a weak cipher is in used.
• Flagged cipher is ssh-rsa or another sha1 based cipher.
• When customer runs a scan for vulnerability, they might get "SSH Weak Message Authentication Code Algorithms" and/or description with "The SSH server supports cryptographically weak Hash-based message authentication" 

Environment

VMware NSX-T Data Center

Resolution

This issue will be fixed in future product release.

Workaround:

The following workaround is to be applied on every NSX appliance:

1. SSH to the appliance as "root".

2. Create a copy of sshd_config file:

cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

3. In section "# Cipher and MAC algorithms", use vi, or vim editor to edit the content as shown below:

Before change:

grep "Cipher and MAC" -A3 /etc/ssh/sshd_config-bak
# Cipher and MAC algorithms
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,[email protected],[email protected],[email protected]
MACs [email protected],[email protected],hmac-sha2-256,hmac-sha2-512
KexAlgorithms curve25519-sha256,[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-ssha512,diffie-hellman-group14-sha256

 

After change:

grep "Cipher and MAC" -A3 /etc/ssh/sshd_config
# Cipher and MAC algorithms
Ciphers [email protected],[email protected]
MACs [email protected],[email protected]
KexAlgorithms ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521

 

4. Get appliance os version from /etc/os-release file.

 

If appliance os is ubuntu 18.04 then add below line (hostkeyalgorithms option) at the end of sshd_config file -

 

hostkeyalgorithms [email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521

If appliance os is ubuntu 20 or greater then add below line (hostkeyalgorithms option) at the end of sshd_config file -

hostkeyalgorithms [email protected],[email protected],[email protected],[email protected],[email protected],[email protected],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[email protected],[email protected]

Add below 'hostkey' options for all ubuntu OS at the end of sshd_config file -

hostkey /etc/ssh/ssh_host_ecdsa_key
hostkey /etc/ssh/ssh_host_ed25519_key

5. Restart sshd service:

# service sshd restart

6. Review available cipheres:

# sshd -T | egrep -i 'cipher|kexalgorithms|Macs|hostkey'

Note: If your SSH client (e.g. PuTTY) throws error similar to:
"Couldn't agree a client-to-server cipher (available: [email protected],[email protected])"
Please update your SSH client/PuTTY to the latest available version (e.g. client supporting the ciphers available on NSX appliances).

Additional Information

Impact/Risks:
Security scanner may flag this as security vulnerability.