cis-ubuntu2004-v300-5-1-5

DevOps & Security

Ensure sshd Banner 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-v300-5-1-5/SKILL.md

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Ensure sshd Banner is configured (Automated)

Profile Applicability

Level 1 - Server
Level 1 - Workstation

Description

The Banner parameter specifies a file whose contents must be sent to the remote user before authentication is permitted. By default, no banner is displayed.

Rationale

Banners are used to warn connecting users of the particular site's policy regarding connection. Presenting a warning message prior to the normal user login may assist the prosecution of trespassers on the computer system.

Audit Procedure

Command Line

Run the following command to verify Banner is set:

# sshd -T | grep -Pi -- '^\h*banner\h+\/\H+'

Example:

banner /etc/issue.net

IF - Match set statements are used in your environment, specify the connection parameters to use for the -T extended test mode and run the audit to verify the setting is not incorrectly configured in a match block. Example additional audit needed for a match block for the user sshuser:

# sshd -T -C user=sshuser | grep -Pi -- '^\h*banner\h+\/\H+'

Run the following command and verify that the contents or the file being called by the Banner argument match site policy:

# [ -e "$(sshd -T | awk '$1 == "banner" {print $2}')" ] && cat "$(sshd -T | awk '$1 == "banner" {print $2}')"

Run the following command and verify no results are returned:

# grep -Psi -- "(\\v|\\r|\\m|\\s)\b$(grep -Pi -- '^ID=' /etc/os-release | cut -d= -f2 | sed -e 's/"//g')\b" "$(sshd -T | awk '$1 == "banner" {print $2}')"

Expected Result

Banner should be set to a valid file path (e.g., /etc/issue.net). The banner content should match site policy and not contain \m, \r, \s, \v or references to the OS platform.

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set the Banner parameter above any Include and Match entries as follows:

Banner /etc/issue.net

Note: First occurrence of a option takes precedence, Match set statements withstanding. 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. Edit the file being called by the Banner argument with the appropriate contents according to your site policy, remove any instances of \m, \r, \s, \v or references to the OS platform. Example:

# printf '%s\n' "Authorized users only. All activity may be monitored and reported." > "$(sshd -T | awk '$1 == "banner" {print $2}')"

References

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: TA0001, TA0007 | M1035