Cis Gcp Cos 3.2.5

---
name: cis-gcp-cos-3.2.5
description: "Ensure broadcast ICMP requests are ignored"
category: cis-gcp-cos
version: "1.2.0"
author: cyberstrike-official
tags: [cis, gcp, container-optimized-os, network, sysctl, icmp]
cis_id: "3.2.5"
cis_benchmark: "CIS Google Container-Optimized OS Benchmark v1.2.0"
tech_stack: [gcp, linux]
cwe_ids: []
chains_with: []
prerequisites: []
severity_boost: {}
---

# 3.2.5 Ensure broadcast ICMP requests are ignored (Automated)

## 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:

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

## Expected Result

The sysctl command should return a value of `1` and the grep command should confirm the setting is persisted in the sysctl configuration files.

## Remediation

Set the following parameters 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:

```bash
# sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
# sysctl -w net.ipv4.route.flush=1
```

`/etc` is stateless on Container-Optimized OS. Therefore, `/etc` cannot be used to make these changes persistent across reboots. The steps mentioned above needs to be performed after every boot.

## CIS Controls

| Controls Version | Control                                                                                                                                                                                                                         | IG 1 | IG 2 | IG 3 |
| ---------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---- | ---- | ---- |
| v8               | **12.2 Establish and Maintain a Secure Network Architecture** - Establish and maintain a secure network architecture. A secure network architecture must address segmentation, least privilege, and availability, at a minimum. |      | X    | X    |
| v7               | **5.1 Establish Secure Configurations** - Maintain documented, standard security configuration standards for all authorized operating systems and software.                                                                     | X    | X    | X    |

## Profile

- Level 1 - Server