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cis-ubuntu2004-v300-5-1-15

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Ensure sshd MACs are configured

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_20.04_LTS_Benchmark_v3.0.0/cis-ubuntu2004-v300-5-1-15/SKILL.md

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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
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

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,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

Default Value

MACs umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1

References

  1. https://nvd.nist.gov/vuln/detail/CVE-2023-48795
  2. More information on SSH downgrade attacks can be found here: http://www.mitls.org/pages/attacks/SLOTH
  3. SSHD_CONFIG(5)
  4. NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5

CIS Controls

v8 - 3.10 Encrypt Sensitive Data in Transit Encrypt sensitive data in transit. Example implementations can include: Transport Layer Security (TLS) and Open Secure Shell (OpenSSH).

v7 - 14.4 Encrypt All Sensitive Information in Transit Encrypt all sensitive information in transit.

v7 - 16.5 Encrypt Transmittal of Username and Authentication Credentials Ensure that all account usernames and authentication credentials are transmitted across networks using encrypted channels.

MITRE ATT&CK Mappings: T1040, T1040.000, T1557, T1557.000 | TA0006 | M1041