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cis-ubuntu1604-v200-3-5-3-3-2

DevOps & Security
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Ensure ip6tables loopback traffic 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_16.04_LTS_Benchmark_v2.0.0/cis-ubuntu1604-v200-3-5-3-3-2/SKILL.md

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CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.5.3.3.2

Profile

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

Description

Configure the loopback interface to accept traffic. Configure all other interfaces to deny traffic to the loopback network (::1).

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

Loopback traffic is generated between processes on machine and is typically critical to operation of the system. The loopback interface is the only place that loopback network (::1) traffic should be seen, all other interfaces should ignore traffic on this network as an anti-spoofing measure.

Audit Procedure

Command Line

Run the following commands and verify output includes the listed rules in order (packet and byte counts may differ):

ip6tables -L INPUT -v -n

ip6tables -L OUTPUT -v -n

OR verify IPv6 is disabled. Run the following script. Output will confirm if IPv6 is disabled on the system.

#!/usr/bin/bin/bash

output=""
grubfile="$(find -L /boot -name 'grub.cfg' -type f)"
[ -f "$grubfile" ] && ! grep "^\s*linux" "$grubfile" | grep -vq ipv6.disable=1 && output="ipv6 disabled in grub config"
grep -Eqs "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && grep -Eqs "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" /etc/sysctl.conf /etc/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /run/sysctl.d/*.conf && sysctl net.ipv6.conf.all.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.all\.disable_ipv6\s*=\s*1\b" && sysctl net.ipv6.conf.default.disable_ipv6 | grep -Eq "^\s*net\.ipv6\.conf\.default\.disable_ipv6\s*=\s*1\b" && output="ipv6 disabled in sysctl config"
[ -n "$output" ] && echo -e "\n$output" || echo -e "\n*** IPv6 is enabled on the system ***"

Expected Result

Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all      lo     *       ::/0                 ::/0
    0     0 DROP       all      *      *       ::1                  ::/0

Chain OUTPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all      *      lo      ::/0                 ::/0

Remediation

Command Line

Run the following commands to implement the loopback rules:

ip6tables -A INPUT -i lo -j ACCEPT
ip6tables -A OUTPUT -o lo -j ACCEPT
ip6tables -A INPUT -s ::1 -j DROP

References

None

CIS Controls

Version 7

9.4 Apply Host-based Firewalls or Port Filtering - Apply host-based firewalls or port filtering tools on end systems, with a default-deny rule that drops all traffic except those services and ports that are explicitly allowed.