SSH Security Made Easy: An Introduction to ssh-audit

ssh-audit is a powerful tool designed to help you assess the security of your SSH servers (and clients!). It provides detailed information about the server’s configuration, supported algorithms, and potential vulnerabilities. In this guide, I’ll walk you through the steps to install ssh-audit and run your first security tests. Secure SSH configuration made easy.

Installation on Linux

Clone the Repository: Open your terminal and clone the ssh-audit repository from GitHub:
git clone https://github.com/jtesta/ssh-audit.git
Navigate to the Directory: Change to the ssh-audit directory:
cd ssh-audit
Install Dependencies: Ensure you have Python installed on your system. If not, install it using your package manager. For example, on Ubuntu:
sudo apt-get install python3

Installation on macOS

To install ssh-audit , run:
brew install ssh-audit
(You have already Brew installed, right ?)

Please check the ssh-audit url for many other setup options (Docker,Windows,etc.)

Test the SSH-Server against vulnerabilities

execute ssh-audit <hostname>
Replace <hostname> with the IP address or domain name of the SSH server you want to audit.

Example of Ubuntu’s 24.04 LTS default SSHD setup:

(if you add the -l warn switch you just get the vulnerabilities presented)

Interpreting the Results: ssh-audit will provide a detailed report of the server’s configuration, including supported key exchange algorithms, encryption ciphers, and MAC algorithms. Look for any warnings or recommendations to improve your server’s security.

Remediation

After running ssh-audit and identifying potential vulnerabilities or weak configurations in your SSH server, it’s important to take steps to remediate these issues. Below are examples of how to apply them:

Example for Ubuntu 24.04.1 LTS:

(Note: This is just an example. The example eliminates vulnerabilities for the SSH-daemon, but it can well be that this snippet does not fit for your setup. Handle with care)

This snippet creates a configuration file (51-ssh-harden_202412.conf) in directory /etc/ssh/sshd_config.d/ with the specified settings to enhance the security of your SSH server.

echo -e "\n# Restrict key exchange, cipher, and MAC algorithms
# Key Exchange Algorithms
KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256

# Ciphers
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

# Message Authentication Codes (MACs)
MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com

# Host Key Algorithms
HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com" > /etc/ssh/sshd_config.d/51-ssh-harden_202412.conf

(SSHD restart required)

Example for RHEL 9.4

(Note: This is just an example. This example eliminates vulnerabilities for the SSH-daemon, but it can well be that this snippet does not fit for your setup. Handle with care)

# Backup the original OpenSSH server configuration file 
cp /etc/crypto-policies/back-ends/opensshserver.config /etc/crypto-policies/back-ends/opensshserver.config.orig
# Update the OpenSSH server configuration with specific cryptographic policies
echo -e "
# Ciphers: Specifies the encryption algorithms used to secure the SSH session
Ciphers=aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

# Message Authentication Codes (MACs): Defines the algorithms used to ensure data integrity and authenticity
MACs=hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com

# GSSAPI Key Exchange Algorithms: Specifies the algorithms used for GSSAPI key exchange
GSSAPIKexAlgorithms=gss-curve25519-sha256-

# Key Exchange Algorithms (KexAlgorithms): Lists the algorithms used for key exchange during the SSH handshake
KexAlgorithms=curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256

# Host Key Algorithms: Lists the algorithms used for verifying the server's host key
HostKeyAlgorithms=ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512

# Public Key Accepted Key Types: Specifies the types of public keys accepted for authentication
PubkeyAcceptedKeyTypes=ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512
" > /etc/crypto-policies/back-ends/opensshserver.config

(SSHD restart required)

Proof the remediation

run ssh-audit again!

Example-output after remediation:

How can I test if my SSH-Client is not vulnerable ?

If you run ssh-audit with the switch -c it creates an ssh-service on port 2222 and audits every connection attempt:

output after the login-attempt (ssh 127.0.0.1 -p 2222)

➜  ~ ssh-audit -c
# general
(gen) client IP: 127.0.0.1
(gen) banner: SSH-2.0-OpenSSH_9.8
(gen) software: OpenSSH 9.8
(gen) compression: enabled (zlib@openssh.com, zlib)

# key exchange algorithms
(kex) sntrup761x25519-sha512@openssh.com           -- [info] available since OpenSSH 8.5
                                                   `- [info] default key exchange from OpenSSH 9.0 to 9.8
                                                   `- [info] hybrid key exchange based on post-quantum resistant algorithm and proven conventional X25519 algorithm
(kex) curve25519-sha256                            -- [info] available since OpenSSH 7.4, Dropbear SSH 2018.76
                                                   `- [info] default key exchange from OpenSSH 7.4 to 8.9
(kex) curve25519-sha256@libssh.org                 -- [info] available since OpenSSH 6.4, Dropbear SSH 2013.62
(kex) diffie-hellman-group-exchange-sha256         -- [info] available since OpenSSH 4.4
(kex) diffie-hellman-group16-sha512                -- [info] available since OpenSSH 7.3, Dropbear SSH 2016.73
(kex) diffie-hellman-group18-sha512                -- [info] available since OpenSSH 7.3
(kex) ext-info-c                                   -- [info] available since OpenSSH 7.2
                                                   `- [info] pseudo-algorithm that denotes the peer supports RFC8308 extensions
(kex) kex-strict-c-v00@openssh.com                 -- [info] pseudo-algorithm that denotes the peer supports a stricter key exchange method as a counter-measure to the Terrapin attack (CVE-2023-48795)

# host-key algorithms
(key) ssh-ed25519-cert-v01@openssh.com             -- [info] available since OpenSSH 6.5
(key) sk-ssh-ed25519-cert-v01@openssh.com          -- [info] available since OpenSSH 8.2
(key) rsa-sha2-512-cert-v01@openssh.com            -- [info] available since OpenSSH 7.8
(key) rsa-sha2-256-cert-v01@openssh.com            -- [info] available since OpenSSH 7.8
(key) ssh-ed25519                                  -- [info] available since OpenSSH 6.5, Dropbear SSH 2020.79
(key) sk-ssh-ed25519@openssh.com                   -- [info] available since OpenSSH 8.2
(key) rsa-sha2-512                                 -- [info] available since OpenSSH 7.2
(key) rsa-sha2-256                                 -- [info] available since OpenSSH 7.2, Dropbear SSH 2020.79

# encryption algorithms (ciphers)
(enc) chacha20-poly1305@openssh.com                -- [info] available since OpenSSH 6.5, Dropbear SSH 2020.79
                                                   `- [info] default cipher since OpenSSH 6.9
(enc) aes128-ctr                                   -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
(enc) aes192-ctr                                   -- [info] available since OpenSSH 3.7
(enc) aes256-ctr                                   -- [info] available since OpenSSH 3.7, Dropbear SSH 0.52
(enc) aes128-gcm@openssh.com                       -- [info] available since OpenSSH 6.2
(enc) aes256-gcm@openssh.com                       -- [info] available since OpenSSH 6.2

# message authentication code algorithms
(mac) umac-128-etm@openssh.com                     -- [info] available since OpenSSH 6.2
(mac) hmac-sha2-256-etm@openssh.com                -- [info] available since OpenSSH 6.2
(mac) hmac-sha2-512-etm@openssh.com                -- [info] available since OpenSSH 6.2


# algorithm recommendations (for OpenSSH 9.8)
(rec) -ecdh-sha2-nistp256                          -- kex algorithm to remove
(rec) -ecdh-sha2-nistp384                          -- kex algorithm to remove
(rec) -ecdh-sha2-nistp521                          -- kex algorithm to remove
(rec) -ecdsa-sha2-nistp256                         -- key algorithm to remove
(rec) -ecdsa-sha2-nistp256-cert-v01@openssh.com    -- key algorithm to remove
(rec) -ecdsa-sha2-nistp384                         -- key algorithm to remove
(rec) -ecdsa-sha2-nistp384-cert-v01@openssh.com    -- key algorithm to remove
(rec) -ecdsa-sha2-nistp521                         -- key algorithm to remove
(rec) -ecdsa-sha2-nistp521-cert-v01@openssh.com    -- key algorithm to remove
(rec) -hmac-sha1                                   -- mac algorithm to remove
(rec) -hmac-sha1-etm@openssh.com                   -- mac algorithm to remove
(rec) -sk-ecdsa-sha2-nistp256-cert-v01@openssh.com -- key algorithm to remove
(rec) -sk-ecdsa-sha2-nistp256@openssh.com          -- key algorithm to remove
(rec) -diffie-hellman-group14-sha256               -- kex algorithm to remove
(rec) -hmac-sha2-256                               -- mac algorithm to remove
(rec) -hmac-sha2-512                               -- mac algorithm to remove
(rec) -umac-128@openssh.com                        -- mac algorithm to remove
(rec) -umac-64-etm@openssh.com                     -- mac algorithm to remove
(rec) -umac-64@openssh.com                         -- mac algorithm to remove

Make your SSH-communication more secure, if not the SSH-Service opens an attack surface for uninvited visitors.
Secure SSH configuration is Key!

Consider other additional security-steps like:
Secure your SSH communication with certificates
Lab setup: Secure your SSH communication with certificates
Fail2Ban: ban hosts that cause multiple authentication errors
…
..
.