cis-ubuntu2004-v300-5-1-18

DevOps & Security

Ensure sshd MaxStartups is 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-v300-5-1-18/SKILL.md

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Ensure sshd MaxStartups is configured (Automated)

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Description

The MaxStartups parameter specifies the maximum number of concurrent unauthenticated connections to the SSH daemon.

Rationale

To protect a system from denial of service due to a large number of pending authentication connection attempts, use the rate limiting function of MaxStartups to protect availability of sshd logins and prevent overwhelming the daemon.

Audit Procedure

Command Line

Run the following command to verify MaxStartups is 10:30:60 or more restrictive:

# sshd -T | awk '$1 ~ /^\s*maxstartups/{split($2, a, ":");if(a[1] > 10 || a[2] > 30 || a[3] > 60) print $0}'

Nothing should be returned.

Expected Result

Nothing should be returned (MaxStartups is 10:30:60 or more restrictive).

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set the MaxStartups parameter to 10:30:60 or more restrictive above any Include entries as follows:

MaxStartups 10:30:60

Note: First occurrence of a 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.

Default Value

MaxStartups 10:30:100

References

SSHD_CONFIG(5)
NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5

CIS Controls

v8 - 0.0 Explicitly Not Mapped

v7 - 0.0 Explicitly Not Mapped

MITRE ATT&CK Mappings: T1499, T1499.002 | TA0040