cis-ubuntu1804-v220-5-2-1-4

skill_id	cis-ubuntu1804-v220-5-2-1-4
name	cis-ubuntu1804-v220-5-2-1-4
description	Configure the audit_backlog_limit kernel parameter to prevent audit event loss
version	2.2.0
category	cis-logging
tags	["cis","ubuntu","linux","ubuntu-18.04","auditing","auditd"]
profile_applicability	["Level 2 - Server","Level 2 - Workstation"]
severity_boost	{}

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:

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:

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