SV-257996r1134915: RHEL 9 must be configured so that all network connections associated with SSH traffic are terminated after 10 minutes of becoming unresponsive. | Red Hat Enterprise Linux 9 STIG SCAP Benchmark

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Assessment Title

Rule Text

RHEL 9 must be configured so that all network connections associated with SSH traffic are terminated after 10 minutes of becoming unresponsive.

Description

Discussion Text

Terminating an unresponsive SSH session within a short time period reduces the window of opportunity for unauthorized personnel to take control of a management session enabled on the console or console port that has been left unattended. In addition, quickly terminating an idle SSH session will also free up resources committed by the managed network element. Terminating network connections associated with communications sessions includes, for example, deallocating associated TCP/IP address/port pairs at the operating system level and deallocating networking assignments at the application level if multiple application sessions are using a single operating system-level network connection. This does not mean the operating system terminates all sessions or network access; it only ends the unresponsive session and releases the resources associated with that session. RHEL 9 utilizes /etc/ssh/sshd_config for configurations of OpenSSH. Within the sshd_config, the product of the values of "ClientAliveInterval" and "ClientAliveCountMax" are used to establish the inactivity threshold. The "ClientAliveInterval" is a timeout interval in seconds, after which if no data has been received from the client, sshd will send a message through the encrypted channel to request a response from the client. The "ClientAliveCountMax" is the number of client alive messages that may be sent without sshd receiving any messages back from the client. If this threshold is met, sshd will disconnect the client. For more information on these settings and others, refer to the sshd_config man pages. Satisfies: SRG-OS-000126-GPOS-00066, SRG-OS-000163-GPOS-00072, SRG-OS-000279-GPOS-00109, SRG-OS-000395-GPOS-00175

How this relates to the STIG checklist: STIG vs SCAP benchmark.

IA Controls

NIST SP 800-53

Not mappedCCI-001133Not mappedCCI-002361Not mappedCCI-002891

References

DPMS Target Red Hat Enterprise Linux 9 DISA DPMS Target Red Hat Enterprise Linux 9 5551

SCC Automation Reference

The reconstructed evaluation script behind this rule's OVAL check(s) — every module a compound check combines, with this rule's own concrete values and DISA's SCC tool's real per-module mechanism curated from static teardown of SCC releases.

#!/bin/bash
# Reconstructed from oval:mil.disa.stig.rhel9os:def:257996
# RHEL 9 must be configured so that all network connections associated with SSH traffic are terminated after 10 minutes of becoming unresponsive.
# Per-module read mechanism curated from static teardown of DISA's SCC tool
# (SCAP Compliance Checker) -- see the SCC Automation Reference above.

# /etc/ssh/sshd_config or /etc/ssh/sshd_config.d/.* :ClientAliveInterval == 600
# textfilecontent54_object (textfilecontent54_test)
test_1() {
  ( ( [ "$(sed -En 's/^\s*(?i)ClientAliveInterval(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$/\1/p' '/etc/ssh/sshd_config')" -le 600 ] ) && ( [ "$(sed -En 's/^\s*(?i)ClientAliveInterval(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$/\1/p' '/etc/ssh/sshd_config')" -gt 0 ] ) ) || ( ( [ "$(grep -rhP --include='*' '^\s*(?i)ClientAliveInterval(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$' '/etc/ssh/sshd_config.d' | sed -En 's/^\s*(?i)ClientAliveInterval(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$/\1/p')" -le 600 ] ) && ( [ "$(grep -rhP --include='*' '^\s*(?i)ClientAliveInterval(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$' '/etc/ssh/sshd_config.d' | sed -En 's/^\s*(?i)ClientAliveInterval(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$/\1/p')" -gt 0 ] ) )
}

if test_1; then
  echo "PASS"
else
  echo "FAIL"
fi

Assessment Script Powered by AISandbox-executed

A standalone Shell script to assess this control automatically. Review before running it in your environment.

Syntax-checked and actually executed in an isolated sandbox (not the real system this checks) — confirmed to run without errors.

#!/bin/sh
# oval:mil.disa.stig.rhel9os:def:257996
if [ ! -f "/etc/ssh/sshd_config" ] && [ ! -f "/etc/ssh/sshd_config.d/" ]; then echo "PASS"; exit 0; fi
if [ -f "/etc/ssh/sshd_config" ]; then
    grep -q "^ClientAliveInterval[[:space:]][0-9]\{1,3\}" /etc/ssh/sshd_config && grep -Po "^ClientAliveInterval[[:space:]][0-9]\{1,3\}" /etc/ssh/sshd_config | grep -q "600" && echo "PASS"; exit 0
fi
if [ -d "/etc/ssh/sshd_config.d" ]; then
    for file in /etc/ssh/sshd_config.d/*; do
        if [ -f "$file" ]; then
            grep -q "^ClientAliveInterval[[:space:]][0-9]\{1,3\}" "$file" && grep -Po "^ClientAliveInterval[[:space:]][0-9]\{1,3\}" "$file" | grep -q "600" && echo "PASS"; exit 0
        fi
    done
fi
echo "FAIL"; exit 1

Remediation

Note: This setting must be applied in conjunction with RHEL-09-255095 to function correctly.

Configure the SSH server to terminate a user session automatically after the SSH client has been unresponsive for 10 minutes.

Modify or append the following lines in the "/etc/ssh/sshd_config" or in a file in "/etc/ssh/sshd_config.d":

ClientAliveInterval 600

For the changes to take effect, the SSH daemon must be restarted.

$ sudo systemctl restart sshd.service