# Cis Ubuntu2004 V300 3 3 6

> Ensure secure icmp redirects are not accepted

- Skill: `cyberstrikeus/cis-ubuntu2004-v300-3-3-6` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu2004-v300-3-3-6`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu2004-v300-3-3-6/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/cis-ubuntu2004-v300-3-3-6

---


# 3.3.6 Ensure secure icmp redirects are not accepted (Automated)

## Profile

- Level 1 - Server
- Level 1 - Workstation

## Description

Secure ICMP redirects are the same as ICMP redirects, except they come from gateways listed on the default gateway list. It is assumed that these gateways are known to your system, and that they are likely to be secure.

## Rationale

It is still possible for even known gateways to be compromised. Setting `net.ipv4.conf.all.secure_redirects` and `net.ipv4.conf.default.secure_redirects` to 0 protects the system from routing table updates by possibly compromised known gateways.

## Impact

None expected.

## Audit Procedure

### Command Line

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

- `net.ipv4.conf.all.secure_redirects` is set to 0
- `net.ipv4.conf.default.secure_redirects` is set to 0

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 If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the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 both `net.ipv4.conf.all.secure_redirects` and `net.ipv4.conf.default.secure_redirects` set to 0.

## Remediation

### Command Line

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

- `net.ipv4.conf.all.secure_redirects = 0`
- `net.ipv4.conf.default.secure_redirects = 0`

Example:

```bash
# printf '%s\n' "net.ipv4.conf.all.secure_redirects = 0" "net.ipv4.conf.default.secure_redirects = 0" >>
/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.conf.all.secure_redirects=0
  sysctl -w net.ipv4.conf.default.secure_redirects=0
  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.conf.all.secure_redirects = 1

net.ipv4.conf.default.secure_redirects = 1

## References

1. NIST SP 800-53 Rev. 5: CM-1, CM-2, CM-6, CM-7, IA-5

## 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    |