# Cis Ubuntu1804 V220 5 2 1 4

> Configure the audit_backlog_limit kernel parameter to prevent audit event loss

- Skill: `cyberstrikeus/cis-ubuntu1804-v220-5-2-1-4` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cyberstrikeus/cis-ubuntu1804-v220-5-2-1-4`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cyberstrikeus/cis-ubuntu1804-v220-5-2-1-4/raw
- Safety review: CAUTION (external: skill-scanner FAIL, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: cyberstrikeus (https://skillmd.com/u/cyberstrikeus)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cyberstrikeus/cis-ubuntu1804-v220-5-2-1-4

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# CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

## Control 5.2.1.4 - Ensure audit_backlog_limit is sufficient (Automated)

### Profile Applicability

- Level 2 - Server
- Level 2 - Workstation

### Description

In the kernel-level audit subsystem, a socket buffer queue is used to hold audit events. Whenever a new audit event is received, it is logged and prepared to be added to this queue.

The kernel boot parameter `audit_backlog_limit=N`, with N representing the amount of messages, will ensure that a queue cannot grow beyond a certain size. If an audit event is logged which would grow the queue beyond this limit, then a failure occurs and is handled according to the system configuration.

### Rationale

If an audit event is logged which would grow the queue beyond the `audit_backlog_limit`, then a failure occurs, auditd records will be lost, and potential malicious activity could go undetected.

### Audit

Run the following command and verify the `audit_backlog_limit=` parameter is set:

```bash
find /boot -type f -name 'grub.cfg' -exec grep -Ph -- '^\h*linux\b' {} + | grep -Pv 'audit_backlog_limit=\d+\b'
```

**Expected Output:**
Nothing should be returned.

### Remediation

Edit `/etc/default/grub` and add `audit_backlog_limit=N` to `GRUB_CMDLINE_LINUX`. The recommended size for N is 8192 or larger.

**Example:**

```
GRUB_CMDLINE_LINUX="audit_backlog_limit=8192"
```

Run the following command to update the grub2 configuration:

```bash
update-grub
```

### Default Value

If `audit_backlog_limit` is not set, the system defaults to `audit_backlog_limit=64`

### References

- NIST SP 800-53 Rev. 5: AU-2, AU-3, AU-12

### CIS Controls

- v8 8.2 Collect Audit Logs - Collect audit logs. Ensure that logging, per the enterprise's audit log management process, has been enabled across enterprise assets.
- v7 6.2 Activate audit logging - Ensure that local logging has been enabled on all systems and networking devices.
- v7 6.3 Enable Detailed Logging - Enable system logging to include detailed information such as an event source, date, user, timestamp, source addresses, destination addresses, and other useful elements.

### MITRE ATT&CK Mappings

- Techniques: T1562, T1562.001
- Tactics: TA0005 (Defense Evasion)
- Mitigations: M1028 (Operating System Configuration), M1047 (Audit)

### Benchmark

CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

### Automation Status

Automated