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:
# 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:
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:
# 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:
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:
# 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
# Search for kernel module loading events
ausearch -k kernel_module_load -ts this-month
# Search for processes executed from /tmp (common attack staging)
ausearch -k exec_from_tmp -ts this-week
# Search for SSH key modifications
ausearch -k ssh_key_tampering -ts this-month
# Search for a specific event by audit event ID
ausearch -a 12345
# Search events in a specific time range
ausearch -ts 03/15/2026 08:00:00 -te 03/15/2026 18:00:00
# Interpret syscall numbers and format output readably
ausearch -k credential_access -i -ts today
Step 4: Generate Summary Reports with aureport
Use aureport to produce aggregate summaries for triage:
# Summary of all authentication events
aureport -au -ts this-week --summary
# Report of all failed events (login, access, etc.)
aureport --failed --summary -ts today
# Report of executable runs
aureport -x --summary -ts today
# Report of all anomaly events (segfaults, promiscuous mode, etc.)
aureport --anomaly -ts this-week
# Report of file access events
aureport -f --summary -ts today
# Report of all events by key (maps to your custom rule keys)
aureport -k --summary -ts this-month
# Report of all system calls
aureport -s --summary -ts today
# Report of events grouped by user
aureport -u --summary -ts this-week
# Detailed time-based event report for timeline building
aureport -ts 03/15/2026 08:00:00 -te 03/15/2026 18:00:00 --summary
Step 5: Reconstruct the Attack Timeline
Combine ausearch queries to build a chronological narrative:
# Step 5a: Identify the initial access timestamp
ausearch -m USER_LOGIN -ua 0 --success yes -ts this-week -i | head -50
# Step 5b: Trace what the attacker did after gaining access
# Get all events from the compromised account within the incident window
ausearch -ua <UID> -ts "03/15/2026 14:00:00" -te "03/15/2026 18:00:00" -i \
| aureport -f -i
# Step 5c: Extract all commands executed during the incident window
ausearch -m EXECVE -ts "03/15/2026 14:00:00" -te "03/15/2026 18:00:00" -i
# Step 5d: Check for persistence mechanisms installed
ausearch -k cron_persistence -ts "03/15/2026 14:00:00" -i
ausearch -k ssh_key_tampering -ts "03/15/2026 14:00:00" -i
# Step 5e: Check for lateral movement (outbound connections)
ausearch -k network_connection -ts "03/15/2026 14:00:00" -i
Step 6: Forward Audit Logs to SIEM
Configure audisp-remote or auditbeat to ship logs to a central SIEM for correlation:
# Option A: Using audisp-remote plugin
# Edit /etc/audit/plugins.d/au-remote.conf
active = yes
direction = out
path = /sbin/audisp-remote
type = always
# Configure remote target in /etc/audit/audisp-remote.conf
remote_server = siem.internal.corp
port = 6514
transport = tcp
# Option B: Using Elastic Auditbeat
# Install auditbeat and configure /etc/auditbeat/auditbeat.yml
# Auditbeat reads directly from the kernel audit framework
Key Concepts
| Term | Definition |
|---|
| auditd | The Linux Audit daemon that receives audit events from the kernel and writes them to /var/log/audit/audit.log |
| auditctl | Command-line utility to control the audit system: add/remove rules, check status, set backlog size |
| ausearch | Query tool that searches audit logs by message type, user, file, key, time range, or event ID |
| aureport | Reporting tool that generates aggregate summaries of audit events for triage and compliance |
| audit rule key (-k) | A user-defined label attached to an audit rule, enabling fast filtering of related events with ausearch and aureport |
| syscall auditing | Kernel-level monitoring of system calls (execve, open, connect, ptrace) that captures process and file activity |
| augenrules | Utility that merges all files in /etc/audit/rules.d/ into /etc/audit/audit.rules and loads them into the kernel |
Verification
