cis-ubuntu1604-v200-5-3-14

name	cis-ubuntu1604-v200-5-3-14
description	Ensure only strong Ciphers are used
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","ssh","remote-access"]
cis_id	5.3.14
cis_benchmark	CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 5.3.14

Description

This variable limits the ciphers that SSH can use during communication.

Note: Some organizations may have stricter requirements for approved ciphers. Ensure that ciphers used are in compliance with site policy.

Rationale

Weak ciphers that are used for authentication to the cryptographic module cannot be relied upon to provide confidentiality or integrity, and system data may be compromised.

• The DES, Triple DES, and Blowfish ciphers, as used in SSH, have a birthday bound of approximately four billion blocks, which makes it easier for remote attackers to obtain cleartext data via a birthday attack against a long-duration encrypted session, aka a "Sweet32" attack
• The RC4 algorithm, as used in the TLS protocol and SSL protocol, does not properly combine state data with key data during the initialization phase, which makes it easier for remote attackers to conduct plaintext-recovery attacks against the initial bytes of a stream by sniffing network traffic that occasionally relies on keys affected by the Invariance Weakness, and then using a brute-force approach involving LSB values, aka the "Bar Mitzvah" issue
• The passwords used during an SSH session encrypted with RC4 can be recovered by an attacker who is able to capture and replay the session
• Error handling in the SSH protocol; Client and Server, when using a block cipher algorithm in Cipher Block Chaining (CBC) mode, makes it easier for remote attackers to recover certain plaintext data from an arbitrary block of ciphertext in an SSH session via unknown vectors

Audit Procedure

Command Line

Run the following command and verify the output:

sshd -T -C user=root -C host="$(hostname)" -C addr="$(grep $(hostname) /etc/hosts | awk '{print $1}')" | grep -Ei '^\s*ciphers\s+([^#]+,)?(3des-cbc|aes128-cbc|aes192-cbc|aes256-cbc|arcfour|arcfour128|arcfour256|blowfish-cbc|cast128-cbc|[e-mail])\b'

Nothing should be returned.

Run the following command and verify the output:

grep -Ei '^\s*ciphers\s+([^#]+,)?(3des-cbc|aes128-cbc|aes192-cbc|aes256-cbc|arcfour|arcfour128|arcfour256|blowfish-cbc|cast128-cbc|[e-mail])\b' /etc/ssh/sshd_config

Nothing should be returned.

Remediation

Command Line

Edit the /etc/ssh/sshd_config file add/modify the line to contain a comma separated list of the site approved ciphers.