Set SSH Client Alive Count Max

An XCCDF Rule

Description

The SSH server sends at most ClientAliveCountMax messages during a SSH session and waits for a response from the SSH client. The option ClientAliveInterval configures timeout after each ClientAliveCountMax message. If the SSH server does not receive a response from the client, then the connection is considered unresponsive and terminated. For SSH earlier than v8.2, a ClientAliveCountMax value of 0 causes a timeout precisely when the ClientAliveInterval is set. Starting with v8.2, a value of 0 disables the timeout functionality completely. If the option is set to a number greater than 0, then the session will be disconnected after ClientAliveInterval * ClientAliveCountMax seconds without receiving a keep alive message.

Rationale

This ensures a user login will be terminated as soon as the ClientAliveInterval is reached.

ID: xccdf_org.ssgproject.content_rule_sshd_set_keepalive
Severity: Medium
References: BP28(R32)

1 12 13 14 15 16 18 3 5 7 8

5.5.6

APO13.01 BAI03.01 BAI03.02 BAI03.03 DSS01.03 DSS03.05 DSS05.04 DSS05.05 DSS05.07 DSS05.10 DSS06.03 DSS06.10

3.1.11

CCI-000879 CCI-001133 CCI-002361

164.308(a)(4)(i) 164.308(b)(1) 164.308(b)(3) 164.310(b) 164.312(e)(1) 164.312(e)(2)(ii)

4.3.3.2.2 4.3.3.5.1 4.3.3.5.2 4.3.3.6.1 4.3.3.6.2 4.3.3.6.3 4.3.3.6.4 4.3.3.6.5 4.3.3.6.6 4.3.3.6.7 4.3.3.6.8 4.3.3.6.9 4.3.3.7.2 4.3.3.7.3 4.3.3.7.4 4.3.4.3.3

SR 1.1 SR 1.10 SR 1.2 SR 1.3 SR 1.4 SR 1.5 SR 1.7 SR 1.8 SR 1.9 SR 2.1 SR 6.2

A.12.4.1 A.12.4.3 A.14.1.1 A.14.2.1 A.14.2.5 A.18.1.4 A.6.1.2 A.6.1.5 A.7.1.1 A.9.1.2 A.9.2.1 A.9.2.2 A.9.2.3 A.9.2.4 A.9.2.6 A.9.3.1 A.9.4.1 A.9.4.2 A.9.4.3 A.9.4.4 A.9.4.5

CIP-004-6 R2.2.3 CIP-007-3 R5.1 CIP-007-3 R5.2 CIP-007-3 R5.3.1 CIP-007-3 R5.3.2 CIP-007-3 R5.3.3

AC-12 AC-17(a) AC-2(5) CM-6(a) SC-10

DE.CM-1 DE.CM-3 PR.AC-1 PR.AC-4 PR.AC-6 PR.AC-7 PR.IP-2

Req-8.1.8

8.2.8

SRG-OS-000163-GPOS-00072 SRG-OS-000279-GPOS-00109

CCE-27082-7

4.2.7
Updated: about 1 year ago

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

var_sshd_set_keepalive='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_sshd_set_keepalive" use="legacy"/>'

if [ -e "/etc/ssh/sshd_config" ] ; then
    
    LC_ALL=C sed -i "/^\s*ClientAliveCountMax\s\+/Id" "/etc/ssh/sshd_config"
else
    touch "/etc/ssh/sshd_config"
fi
# make sure file has newline at the end
sed -i -e '$a\' "/etc/ssh/sshd_config"

cp "/etc/ssh/sshd_config" "/etc/ssh/sshd_config.bak"
# Insert at the beginning of the file
printf '%s\n' "ClientAliveCountMax $var_sshd_set_keepalive" > "/etc/ssh/sshd_config"
cat "/etc/ssh/sshd_config.bak" >> "/etc/ssh/sshd_config"
# Clean up after ourselves.
rm "/etc/ssh/sshd_config.bak"

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi

- name: XCCDF Value var_sshd_set_keepalive # promote to variable
  set_fact:
    var_sshd_set_keepalive: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_sshd_set_keepalive" use="legacy"/>
  tags:
    - always
- name: Set SSH Client Alive Count Max
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*ClientAliveCountMax\s+
      state: absent
    check_mode: true
    changed_when: false
    register: dupes

  - name: Deduplicate values from /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*ClientAliveCountMax\s+
      state: absent
    when: dupes.found is defined and dupes.found > 1

  - name: Insert correct line to /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*ClientAliveCountMax\s+
      line: ClientAliveCountMax {{ var_sshd_set_keepalive }}
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CCE-27082-7
  - CJIS-5.5.6
  - NIST-800-171-3.1.11
  - NIST-800-53-AC-12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-2(5)
  - NIST-800-53-CM-6(a)
  - NIST-800-53-SC-10
  - PCI-DSS-Req-8.1.8
  - PCI-DSSv4-8.2.8
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_set_keepalive

Set SSH Client Alive Count Max

Description

Rationale

Remediation - Shell Script

Remediation - Ansible