fix: ssh-audit hardenings

[scop]

Proposed Changes

Brings current ssh-audit output green again, and adds an additional moduli hardening per their recommendations.

# algorithm recommendations (for OpenSSH 9.9)
(rec) !diffie-hellman-group-exchange-sha256 -- kex algorithm to change (increase modulus size to 3072 bits or larger) 
(rec) -curve25519-sha256                    -- kex algorithm to remove 
(rec) [email protected]         -- kex algorithm to remove 
(rec) -diffie-hellman-group16-sha512        -- kex algorithm to remove 
(rec) -diffie-hellman-group18-sha512        -- kex algorithm to remove 

Summary by CodeRabbit

• Security Improvements
  • Enhanced SSH configuration by restricting the moduli file to use stronger cryptographic parameters.
  • Broadened the list of disallowed SSH key exchange algorithms to improve security.

[coderabbitai]

Walkthrough

The changes update the SSH Dockerfile to filter the moduli file for stronger cryptographic parameters and expand the list of disabled key exchange algorithms in the SSH daemon configuration to further restrict weaker or less preferred algorithms.

Changes

File(s)	Change Summary
ssh/Dockerfile	Adds a hardening step to filter /etc/ssh/moduli for entries with a minimum strength of 3071 bits.
ssh/rootfs/etc/ssh/sshd_config	Expands the list of disabled KexAlgorithms to exclude additional key exchange algorithms.

Suggested labels

bugfix

Suggested reviewers

• frenck

Poem

In the burrow where the strong keys dwell,
The moduli were filtered, their stories to tell.
More algorithms banished, the SSH gate tight,
Security’s garden, now guarded at night.
With every hop and every byte,
This bunny ensures your SSH is right! 🐇🔐

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📜 Recent review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 88f43c1 and 54735f6.

📒 Files selected for processing (2)

• ssh/Dockerfile (1 hunks)
• ssh/rootfs/etc/ssh/sshd_config (1 hunks)

🔇 Additional comments (2)

ssh/Dockerfile (1)

144-147: Excellent SSH moduli hardening implementation.

This change correctly implements the ssh-audit recommendation to filter SSH moduli for stronger cryptographic parameters. The awk command filters /etc/ssh/moduli to retain only entries where the 5th field (prime bit length) is >= 3071, which removes weaker Diffie-Hellman groups and ensures stronger key exchange security. The atomic file replacement pattern is also properly implemented.

ssh/rootfs/etc/ssh/sshd_config (1)

26-26:

❓ Verification inconclusive

Verify compatibility impact of aggressive algorithm restrictions.

This change significantly expands the list of disabled key exchange algorithms, including some that are generally considered secure like curve25519-sha256. While this aligns with strict security recommendations from ssh-audit, it may cause compatibility issues with older SSH clients or specific use cases.

Please verify that this restrictive configuration doesn't break legitimate SSH connections by testing with various client versions and scenarios.

---

🏁 Script executed:

#!/bin/bash
# Description: Check what key exchange algorithms remain available after these restrictions

# Find the default KexAlgorithms list in OpenSSH
ssh -Q kex

# Show what algorithms would be disabled by this configuration
echo "Disabled algorithms:"
echo "curve25519-sha256,[email protected],diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521"

Length of output: 626

---

Verify compatibility impact of aggressive algorithm restrictions.

The change in ssh/rootfs/etc/ssh/sshd_config now disables a broad set of KexAlgorithms—including curve25519-sha256 variants and several DH/NIST curves—that are normally enabled by default. Since these algorithms are widely supported and often negotiated by modern SSH clients, this configuration may break connections with certain clients or automated tooling.

To confirm no legitimate use cases are impacted, please:

• On your target SSH server (with the exact OpenSSH version you’re deploying), run:

ssh -Q kex

to list the remaining key-exchange algorithms after these exclusions.
• Verify successful SSH logins (interactively and non-interactively) from representative clients, including older versions and any specialized tooling that relies on default KEX lists.
• If you discover missing support, consider re-enabling specific secure algorithms (e.g. curve25519-sha256) rather than disabling the full default set.

---

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[scop]

Re removing diffie-hellman-group-exchange-sha256 -- I'm not sure what their "increase modulus size to 3072 bits or larger" recommendation for this algorithm exactly means, but at least leaving it in and making the moduli changes did not result the warning going away.

[scop]

Filed jtesta/ssh-audit#334 asking for clarification