cis-ubuntu1804-v220-4-2-11

DevOps & Security

Ensure sshd KexAlgorithms is configured

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Prompt to paste

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_18.04_LTS_Benchmark_v2.2.0/cis-ubuntu1804-v220-4-2-11/SKILL.md

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Copying this prompt does not install or run the skill. Review third-party files before use. Codex skill guide

CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 4.2.11

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Description

Key exchange is any method in cryptography by which cryptographic keys are exchanged between two parties, allowing 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.

Rationale

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 Procedure

Command Line

Run the following command and verify that only approved key exchange algorithms are used:

sshd -T | grep -i kexalgorithms

Expected Result

Verify that output does not contain any of the following weak key exchange algorithms:

diffie-hellman-group1-sha1
diffie-hellman-group14-sha1
diffie-hellman-group-exchange-sha1

Remediation

Command Line

Edit the /etc/ssh/sshd_config file and add/modify the KexAlgorithms line to contain a comma separated list of the site approved key exchange algorithms.

Example:

KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256

References

NIST SP 800-53 Rev. 5: AC-17(2), IA-5(1), SC-13

CIS Controls

v8 - 3.10 Encrypt Sensitive Data in Transit - Encrypt sensitive data in transit.

v7 - 14.4 Encrypt All Sensitive Information in Transit.

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Assessment Status

Automated