analyzing-linux-audit-logs-for-intrusion

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• SKILL.md
• api-reference.md · references/
• agent.py · scripts/

SKILL.md (first lines)

# 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_module -k kernel_module_remove
-w /sbin/insmod -p x -k kernel_module_tool
-w /sbin/modprobe -p x -k kernel_module_tool

# Monitor network socket creation for reverse shells
-a always,exit -F arch=b64 -S socket -F a0=2 -k network_socket_created
-a always,exit -F arch=b64 -S connect -F a0=2 -k network_connection

# Detect cron job modifications (persistence)
-w /etc/crontab -p wa -k cron_persistence
-w /etc/cron.d/ -p wa -k cron_persistence
-w /var/spool/cron/ -p wa -k cron_persistence

# Monitor log deletion or tampering
-w /var/log/ -p wa -k log_tampering
```

Reload rules after editing:

```bash
augenrules --load
auditctl -l | wc -l   # Confirm rule count
```

### Step 3: Search for Intrusion Indicators with ausearch

Use `ausearch` to query the audit log for specific events:

```bash
# Search for all failed login attempts in the last 24 hours
ausearch -m USER_LOGIN --success no -ts recent

# Search for commands executed by a specific user
ausearch -ua 1001 -m EXECVE -ts today

# Search for all file access events on /etc/shadow
ausearch -f /etc/shadow -ts this-week

# Search for privilege escalation via sudo
ausearch -m USER_CMD -ts today

From Joyooosama/dsh-skills-for-forensics, Apache-2.0.

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