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SSH Algorithm Enumeration and Weak Crypto Detection

3 min read Jan 4, 2026

Enumerate KEX, ciphers, MACs, and hostkey algorithms to identify legacy crypto support and hardening gaps.

What you are mapping

This page is about what the SSH service is willing to negotiate. You are building a capability profile that drives decisions:

Legacy support is often the real finding, not a specific exploit. If a server still accepts weak KEX or CBC, it usually means hardening is absent or drifted. It also explains client errors and tells you whether older tooling will work.

Signals that are typically worth documenting:

  • diffie-hellman-group1-sha1

  • SHA1-based MACs like hmac-sha1

  • CBC ciphers

  • legacy hostkey algorithms and weak key sizes

  • RSA host keys under 2048 bits

  • anything indicating SSHv1 support

Algorithm enumeration with Nmap

This gives you the raw advertised lists. It is fast, reliable, and easy to paste into notes.

┌──(kali㉿kali)-[~]
└─$ nmap -p $PORT -vv --script ssh2-enum-algos $TARGET_IP 
PORT   STATE SERVICE
22/tcp open  ssh
| ssh2-enum-algos:
|   kex_algorithms (4)
|       diffie-hellman-group-exchange-sha256
|       diffie-hellman-group-exchange-sha1
|       diffie-hellman-group14-sha1
|       diffie-hellman-group1-sha1
|   server_host_key_algorithms (2)
|       ssh-rsa
|       ssh-dss
|   encryption_algorithms (13)
|       aes128-ctr
|       aes192-ctr
|       aes256-ctr
|       arcfour256
|       arcfour128
|       aes128-cbc
|       3des-cbc
|       blowfish-cbc
|       cast128-cbc
|       aes192-cbc
|       aes256-cbc
|       arcfour
|       [email protected]
|   mac_algorithms (6)
|       hmac-md5
|       hmac-sha1
|       hmac-ripemd160
|       [email protected]
|       hmac-sha1-96
|       hmac-md5-96
|   compression_algorithms (2)
|       none
|_      [email protected]

A common pattern on older appliances is “modern enough to connect” but still advertising group1 and CBC for compatibility. That is usually a hardening finding even if you never weaponize it.

SSHv1 check

SSHv1 is rare. If it is enabled, capture it clearly. That is not “legacy for compatibility”, it is a serious misconfiguration.

┌──(kali㉿kali)-[~]
└─$ nmap -p $PORT --script sshv1 $TARGET_IP 
PORT   STATE SERVICE
22/tcp open  ssh
|_sshv1: Server supports SSHv1

Fast interpretation with ssh-audit

ssh-audit is useful when you want a quick “what is weak here” view and a clean write-up structure for your report notes.

┌──(kali㉿kali)-[~]
└─$ ssh-audit -p $PORT $TARGET_IP

This is also the quickest way to spot policy-level issues like deprecated algorithms being offered, weak host key types, or suspicious server versions.

Handshake-driven confirmation from a real SSH client

If you hit negotiation errors, do not guess. Confirm what fails and what succeeds.

┌──(kali㉿kali)-[~]
└─$ ssh -vvv -p $PORT -o BatchMode=yes -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null $USERNAME@$TARGET_IP exit

When you see errors like “no matching key exchange method found”, that typically means the server is stuck on older KEX choices or your client is refusing legacy algorithms. Confirm the exact algorithm set with ssh2-enum-algos and record it.

Phase boundary note: forcing very specific downgrade combinations to make a connection succeed can stay “enumeration” if you only prove acceptance. Turning that into interception or credential capture work is not enumeration and should be a separate exploitation page.

References