cis-ubuntu2004-v300-5-1-15

name	cis-ubuntu2004-v300-5-1-15
description	Ensure sshd MACs are configured
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","ssh"]
cis_id	5.1.15
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

Ensure sshd MACs are configured (Automated)

Profile Applicability

• Level 1 - Server
• Level 1 - Workstation

Description

This variable limits the types of MAC algorithms that SSH can use during communication.

Notes:

• Some organizations may have stricter requirements for approved MACs.
• Ensure that MACs used are in compliance with site policy.
• The only "strong" MACs currently FIPS 140 approved are:
  • HMAC-SHA1
  • HMAC-SHA2-256
  • HMAC-SHA2-384
  • HMAC-SHA2-512

Rationale

MD5 and 96-bit MAC algorithms are considered weak and have been shown to increase exploitability in SSH downgrade attacks. Weak algorithms continue to have a great deal of attention as a weak spot that can be exploited with expanded computing power. An attacker that breaks the algorithm could take advantage of a MiTM position to decrypt the SSH tunnel and capture credentials and information.

Audit Procedure

Command Line

Run the following command to verify none of the "weak" MACs are being used:

# sshd -T | grep -Pi -- 'macs\h+([^#\n\r]+,)?(hmac-md5|hmac-md5-96|hmac-ripemd160|hmac-sha1-96|umac-64@openssh\.com|hmac-md5-etm@openssh\.com|hmac-md5-96-etm@openssh\.com|hmac-ripemd160-etm@openssh\.com|hmac-sha1-96-etm@openssh\.com|umac-64-etm@openssh\.com|umac-128-etm@openssh\.com)\b'

Nothing should be returned. Note: Review CVE-2023-48795 and verify the system has been patched. If the system has not been patched, review the use of the Encrypt Then Mac (etm) MACs. The following are considered "weak" MACs, and should not be used:

hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-sha1-96
[e-mail]
[e-mail]
[e-mail]
[e-mail]
[e-mail]
[e-mail]
[e-mail]

Expected Result

Nothing should be returned (no weak MACs in use).

Remediation

Command Line

Edit the /etc/ssh/sshd_config file and add/modify the MACs line to contain a comma separated list of the site unapproved (weak) MACs preceded with a - above any Include entries: Example:

MACs -hmac-md5,hmac-md5-96,hmac-ripemd160,hmac-sha1-96,[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail]

• IF - CVE-2023-48795 has not been reviewed and addressed, the following MACs should be added to the exclude list: ,, Note: First occurrence of an option takes precedence. If Include locations are enabled, used, and order of precedence is understood in your environment, the entry may be created in a file in Include location.