Cis Ubuntu2004 V300 3 3 4

---
name: cis-ubuntu2004-v300-3-3-4
description: "Ensure broadcast icmp requests are ignored"
category: cis-networking
version: "3.0.0"
author: cyberstrike-official
tags: [cis, ubuntu, linux, ubuntu-20.04, kernel-parameter]
cis_id: "3.3.4"
cis_benchmark: "CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0"
tech_stack: [ubuntu, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# 3.3.4 Ensure broadcast icmp requests are ignored (Automated)

## Profile

- 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.

## Impact

None expected.

## Audit Procedure

### Command Line

Run the following script to verify the following kernel parameter is set in the running configuration and correctly loaded from a kernel parameter configuration file:

- `net.ipv4.icmp_echo_ignore_broadcasts` is set to 1

Note: kernel parameters are loaded by file and parameter order precedence. The following script observes this precedence as part of the auditing procedure. The parameters being checked may be set correctly in a file. If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the script and by the system during a normal kernel parameter load sequence.

## Expected Result

Audit Result: ** PASS ** with `net.ipv4.icmp_echo_ignore_broadcasts` set to 1.

## Remediation

### Command Line

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

- `net.ipv4.icmp_echo_ignore_broadcasts = 1`

Example:

```bash
# printf '%s\n' "net.ipv4.icmp_echo_ignore_broadcasts = 1" >> /etc/sysctl.d/60-netipv4_sysctl.conf
```

Run the following script to set the active kernel parameters:

```bash
#!/usr/bin/env bash

{
  sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
  sysctl -w net.ipv4.route.flush=1
}
```

Note: If these settings appear in a canonically later file, or later in the same file, these settings will be overwritten.

## Default Value

net.ipv4.icmp_echo_ignore_broadcasts = 1

## References

1. NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5
2. NIST SP 800-53A :: CM-6.1 (iv)
3. RHEL 8 STIG Vul ID: V-230537
4. RHEL 8 STIG Rule ID: SV-230537r858797

## CIS Controls

| Controls Version | Control                                                                         | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------------------------------------- | ---- | ---- | ---- |
| v8               | 4.8 Uninstall or Disable Unnecessary Services on Enterprise Assets and Software |      | x    | x    |
| v7               | 9.2 Ensure Only Approved Ports, Protocols and Services Are Running              |      | x    | x    |