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CIS Ubuntu 14.04 LTS - 3.2.5 Ensure broadcast ICMP requests are ignored

DevOps & Security
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Verify that broadcast ICMP echo and timestamp requests are ignored to prevent Smurf attacks

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Source SKILL.md: https://github.com/CyberStrikeus/CyberStrike/blob/HEAD/.cyberstrike/skill/CIS_benchmarks/Operating_Systems/Ubuntu/CIS_Ubuntu_Linux_14.04_LTS_Benchmark_v2.1.0/cis-ubuntu1404-v210-3-2-5/SKILL.md

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3.2.5 Ensure broadcast ICMP requests are ignored (Scored)

Profile Applicability

  • Level 1 - Server
  • Level 1 - Workstation

Description

Setting net.ipv4.icmp_echo_ignore_broadcasts to 1 will cause the system to ignore all ICMP echo and timestamp requests to broadcast and multicast addresses.

Rationale

Accepting ICMP echo and timestamp requests with broadcast or multicast destinations for your network could be used to trick your host into starting (or participating) in a Smurf attack. A Smurf attack relies on an attacker sending large amounts of ICMP broadcast messages with a spoofed source address. All hosts receiving this message and responding would send echo-reply messages back to the spoofed address, which is probably not routable. If many hosts respond to the packets, the amount of traffic on the network could be significantly multiplied.

Audit Procedure

Run the following commands and verify output matches:

sysctl net.ipv4.icmp_echo_ignore_broadcasts
# Expected: net.ipv4.icmp_echo_ignore_broadcasts = 1

grep "net\.ipv4\.icmp_echo_ignore_broadcasts" /etc/sysctl.conf /etc/sysctl.d/*
# Expected: net.ipv4.icmp_echo_ignore_broadcasts = 1

Expected Result

net.ipv4.icmp_echo_ignore_broadcasts = 1

Remediation

Set the following parameter in /etc/sysctl.conf or a /etc/sysctl.d/* file:

net.ipv4.icmp_echo_ignore_broadcasts = 1

Run the following commands to set the active kernel parameters:

sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
sysctl -w net.ipv4.route.flush=1

Default Value

Not specified.

References

  • CIS Controls: 3 - Secure Configurations for Hardware and Software on Mobile Devices, Laptops, Workstations, and Servers
  • CIS Controls: 11 - Secure Configurations for Network Devices such as Firewalls, Routers and switches

Profile

  • Level 1 - Server
  • Level 1 - Workstation