cis-gcp-cos-5-1-13

name	cis-gcp-cos-5.1.13
description	Ensure only strong Ciphers are used
category	cis-gcp-cos
version	1.2.0
author	cyberstrike-official
tags	["cis","gcp","container-optimized-os","ssh","sshd","sshd-config","encryption","ciphers"]
cis_id	5.1.13
cis_benchmark	CIS Google Container-Optimized OS Benchmark v1.2.0
tech_stack	["gcp","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

5.1.13 Ensure only strong Ciphers are used (Automated)

Description

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

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.

The mm_newkeys_from_blob function in monitor_wrap.c, when an AES-GCM cipher is used, does not properly initialize memory for a MAC context data structure, which allows remote authenticated users to bypass intended ForceCommand and login-shell restrictions via packet data that provides a crafted callback address.

Audit Procedure

Run the following command and verify that output does not contain any of the listed weak ciphers:

# sshd -T | grep ciphers

Weak Ciphers:

3des-cbc
aes128-cbc
aes192-cbc
aes256-cbc
arcfour
arcfour128
arcfour256
blowfish-cbc
cast128-cbc
[e-mail]

Expected Result

Output should not contain any of the listed weak ciphers.

Remediation

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

Ciphers [e-mail],[e-mail],[e-mail],aes256-ctr,aes192-ctr,aes128-ctr

Default Value

ciphers [e-mail],[e-mail],,aes128-ctr,aes192-ctr,aes256-ctr