analyzing-linux-audit-logs-for-intrusion

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---
name: analyzing-linux-audit-logs-for-intrusion
description: 'Uses the Linux Audit framework (auditd) with ausearch and aureport utilities
  to detect intrusion attempts, unauthorized access, privilege escalation, and suspicious
  system activity. Covers audit rule configuration, log querying, timeline reconstruction,
  and integration with SIEM platforms. Activates for requests involving auditd analysis,
  Linux audit log investigation, ausearch queries, aureport summaries, or host-based
  intrusion detection on Linux.

  '
domain: cybersecurity
subdomain: incident-response
tags:
- auditd
- ausearch
- aureport
- linux-security
- intrusion-detection
- HIDS
- forensics
version: 1.0.0
author: mahipal
license: Apache-2.0
nist_csf:
- RS.MA-01
- RS.MA-02
- RS.AN-03
- RC.RP-01
mitre_attack:
- T1059.004
- T1070
- T1548.003
- T1543.002
---

# Analyzing Linux Audit Logs for Intrusion

## When to Use

- Investigating suspected unauthorized access or privilege escalation on Linux hosts
- Hunting for evidence of exploitation, backdoor installation, or persistence mechanisms
- Auditing compliance with security baselines (CIS, STIG, PCI-DSS) that require system call monitoring
- Reconstructing a timeline of attacker actions during incident response
- Detecting file tampering on critical system files such as `/etc/passwd`, `/etc/shadow`, or SSH keys

**Do not use** for network-level intrusion detection; use Suricata or Zeek for network traffic analysis. Auditd operates at the kernel level on individual hosts.

## Prerequisites

- Linux system with `auditd` package installed and the audit daemon running (`systemctl status auditd`)
- Root or sudo access to configure audit rules and query logs
- Audit rules deployed via `/etc/audit/rules.d/*.rules` or loaded with `auditctl`
- Recommended: Neo23x0/auditd ruleset from GitHub for comprehensive baseline coverage
- Familiarity with Linux syscalls (`execve`, `open`, `connect`, `ptrace`, etc.)
- Log storage with sufficient retention (default location: `/var/log/audit/audit.log`)

## Workflow

### Step 1: Verify Audit Daemon Status and Configuration

Confirm the audit system is running and check the current rule set:

```bash
# Check auditd service status
systemctl status auditd

# Show current audit rules loaded in the kernel
auditctl -l

# Show audit daemon configuration
cat /etc/audit/auditd.conf | grep -E "log_file|max_log_file|num_logs|space_left_action"

# Check if the audit backlog is being exceeded (dropped events)
auditctl -s
```

If the backlog limit is being reached, increase it:

```bash
auditctl -b 8192
```

### Step 2: Deploy Intrusion-Focused Audit Rules

Add rules that target common intrusion indicators. Place these in `/etc/audit/rules.d/intrusion.rules`:

```bash
# Monitor credential files for unauthorized reads or modifications
-w /etc/passwd -p wa -k credential_access
-w /etc/shadow -p rwa -k credential_access
-w /etc/gshadow -p rwa -k credential_access
-w /etc/sudoers -p wa -k privilege_escalation
-w /etc/sudoers.d/ -p wa -k privilege_escalation

# Monitor SSH configuration and authorized keys
-w /etc/ssh/sshd_config -p wa -k sshd_config_change
-w /root/.ssh/authorized_keys -p wa -k ssh_key_tampering

# Monitor user and group management commands
-w /usr/sbin/useradd -p x -k user_management
-w /usr/sbin/usermod -p x -k user_management
-w /usr/sbin/groupadd -p x -k user_management

# Detect process injection via ptrace
-a always,exit -F arch=b64 -S ptrace -F a0=0x4 -k process_injection
-a always,exit -F arch=b64 -S ptrace -F a0=0x5 -k process_injection
-a always,exit -F arch=b64 -S ptrace -F a0=0x6 -k process_injection

# Monitor execution of programs from unusual directories
-a always,exit -F arch=b64 -S execve -F exe=/tmp -k exec_from_tmp
-a always,exit -F arch=b64 -S execve -F exe=/dev/shm -k exec_from_shm

# Detect kernel module loading (rootkit installation)
-a always,exit -F arch=b64 -S init_module -S finit_module -k kernel_module_load
-a always,exit -F arch=b64 -S delete_mo