Ensure sshd KexAlgorithms is configured

HighLevel 1Automatedfreebsd

Description

Key exchange is any cryptography method that exchanges cryptographic keys between two parties, allowing the use of a cryptographic algorithm. If the sender and receiver wish to exchange encrypted messages, each must be equipped to encrypt messages to be sent and decrypt messages received. Key exchange methods that are considered weak should be removed. A key exchange method may be weak because too few bits are used or the hashing algorithm is considered too weak. Using weak algorithms could expose connections to man-in-the- middle attacks. Audit: the effective `kexalgorithms` list from `sshd -T` must not contain any weak algorithm listed by CIS.

How it is checked

Audit method: Shell command

shell

command -v sshd >/dev/null 2>&1 || exit 1; out=$(sshd -T 2>/dev/null) || exit 1; line=$(printf '%s\n' "$out" | grep -i '^kexalgorithms '); [ -n "$line" ] || exit 1; printf '%s\n' "$line" | grep -Eiq -- '(^|[ ,])(diffie-hellman-group1-sha1|diffie-hellman-group14-sha1|diffie-hellman-group-exchange-sha1)(,|$)' && echo "weak: $line" || echo compliant

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Edit the /etc/ssh/sshd_config file and add/modify the KexAlgorithms line to contain a comma-separated list of the site unapproved (weak) KexAlgorithms preceded with a - above any Include entries: Example: KexAlgorithms -diffie-hellman-group1-sha1,diffie-hellman-group14-sha1,diffie- hellman-group-exchange-sha1

Properties

Operating systems: Any
Tags: freebsdssh
Reference: CIS FreeBSD 14 Benchmark v1.0.1 - 4.2.11 Ensure sshd KexAlgorithms is configured

Check this host

Run this on a machine you are allowed to audit. The scan is read-only.

shell

tmaegis scan --target freebsd