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cis-ubuntu2004-v300-4-4-3-1

DevOps & Security
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Ensure ip6tables default deny firewall policy

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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-v300-4-4-3-1/SKILL.md

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CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 - Control 4.4.3.1

Profile

  • Level: Level 1 - Server, Level 1 - Workstation
  • Assessment Status: Automated

Description

A default deny all policy on connections ensures that any unconfigured network usage will be rejected.

Note:

  • Changing firewall settings while connected over network can result in being locked out of the system
  • Remediation will only affect the active system firewall, be sure to configure the default policy in your firewall management to apply on boot as well

Rationale

With a default accept policy the firewall will accept any packet that is not configured to be denied. It is easier to allow list acceptable usage than to deny list unacceptable usage.

Audit Procedure

Command Line

Run the following command and verify that the policy for the INPUT, OUTPUT, and FORWARD chains is DROP or REJECT:

ip6tables -L -n

Chain INPUT (policy DROP)
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)
  • OR - Verify IPv6 is disabled: Run the following script. Output will confirm if IPv6 is enabled on the system.
#!/usr/bin/env bash

{
  l_ipv6_enabled="is"
  ! grep -Pqs -- '^\h*0\b' /sys/module/ipv6/parameters/disable && l_ipv6_enabled="is not"
  if sysctl net.ipv6.conf.all.disable_ipv6 | grep -Pqs -- "^\h*net\.ipv6\.conf\.all\.disable_ipv6\h*=\h*1\b" \
  && \
    sysctl net.ipv6.conf.default.disable_ipv6 | grep -Pqs -- "^\h*net\.ipv6\.conf\.default\.disable_ipv6\h*=\h*1\b"; then
      l_ipv6_enabled="is not"
  fi
  echo -e " - IPv6 $l_ipv6_enabled enabled on the system"
}

Expected Result

Chain INPUT (policy DROP)
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)

Or IPv6 is confirmed as disabled.

Remediation

Command Line

  • IF - IPv6 is enabled on your system: Run the following commands to implement a default DROP policy:
ip6tables -P INPUT DROP
ip6tables -P OUTPUT DROP
ip6tables -P FORWARD DROP

Default Value

Not applicable.

References

  1. NIST SP 800-53 Rev. 5: CA-9, SC-7

CIS Controls

v8 - 4.4 Implement and Manage a Firewall on Servers v8 - 4.5 Implement and Manage a Firewall on End-User Devices v7 - 9.4 Apply Host-based Firewalls or Port Filtering

MITRE ATT&CK Mappings: T1562, T1562.004 | TA0011 | M1031, M1037