undermybelt Configuring Host-Based Intrusion Detection
When to Use
Use this skill when:
- Deploying HIDS agents (Wazuh, OSSEC, AIDE) across Windows and Linux endpoints
- Configuring file integrity monitoring (FIM) for compliance (PCI DSS 11.5, NIST SI-7)
- Monitoring system configuration changes, rootkit detection, and security policy violations
- Integrating HIDS alerts with SIEM platforms for centralized monitoring
Do not use this skill for network-based IDS (Suricata, Snort) or for EDR deployment.
Prerequisites
- Wazuh server (manager) deployed and accessible from endpoints
- Administrative access to target endpoints
- Network connectivity: agents to Wazuh manager on port 1514 (TCP/UDP) and 1515 (TCP enrollment)
- Wazuh dashboard (OpenSearch Dashboards) for alert visualization
- Understanding of critical files/directories to monitor per OS
Workflow
Step 1: Install Wazuh Agent
Windows:
# Download and install Wazuh agent
Invoke-WebRequest -Uri "https://packages.wazuh.com/4.x/windows/wazuh-agent-4.9.0-1.msi" `
-OutFile "wazuh-agent.msi"
msiexec /i wazuh-agent.msi /q WAZUH_MANAGER="wazuh-manager.corp.com" `
WAZUH_REGISTRATION_SERVER="wazuh-manager.corp.com" WAZUH_AGENT_GROUP="windows-workstations"
net start WazuhSvc
Linux (Debian/Ubuntu):
curl -s https://packages.wazuh.com/key/GPG-KEY-WAZUH | gpg --dearmor -o /usr/share/keyrings/wazuh.gpg
echo "deb [signed-by=/usr/share/keyrings/wazuh.gpg] https://packages.wazuh.com/4.x/apt/ stable main" \
> /etc/apt/sources.list.d/wazuh.list
apt-get update && apt-get install wazuh-agent -y
sed -i 's/MANAGER_IP/wazuh-manager.corp.com/' /var/ossec/etc/ossec.conf
systemctl daemon-reload && systemctl enable --now wazuh-agent
Step 2: Configure File Integrity Monitoring (FIM)
Edit agent configuration (/var/ossec/etc/ossec.conf or C:\Program Files (x86)\ossec-agent\ossec.conf):
<syscheck>
<!-- Scan frequency: every 12 hours -->
<frequency>43200</frequency>
<scan_on_start>yes</scan_on_start>
<alert_new_files>yes</alert_new_files>
<!-- Linux critical directories -->
<directories check_all="yes" realtime="yes">/etc</directories>
<directories check_all="yes" realtime="yes">/usr/bin</directories>
<directories check_all="yes" realtime="yes">/usr/sbin</directories>
<directories check_all="yes" realtime="yes">/bin</directories>
<directories check_all="yes" realtime="yes">/sbin</directories>
<directories check_all="yes">/boot</directories>
<!-- Windows critical directories -->
<directories check_all="yes" realtime="yes">C:\Windows\System32</directories>
<directories check_all="yes" realtime="yes">C:\Windows\SysWOW64</directories>
<directories check_all="yes" realtime="yes">%PROGRAMFILES%</directories>
<!-- Windows registry monitoring -->
<windows_registry>HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run</windows_registry>
<windows_registry>HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\RunOnce</windows_registry>
<windows_registry>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services</windows_registry>
<!-- Ignore frequently changing files -->
<ignore>/etc/mtab</ignore>
<ignore>/etc/resolv.conf</ignore>
<ignore type="sregex">.log$</ignore>
</syscheck>
Step 3: Configure Rootkit Detection
<rootcheck>
<disabled>no</disabled>
<frequency>43200</frequency>
<rootkit_files>/var/ossec/etc/shared/rootkit_files.txt</rootkit_files>
<rootkit_trojans>/var/ossec/etc/shared/rootkit_trojans.txt</rootkit_trojans>
<system_audit>/var/ossec/etc/shared/system_audit_rcl.txt</system_audit>
<check_dev>yes</check_dev>
<check_files>yes</check_files>
<check_if>yes</check_if>
<check_pids>yes</check_pids>
<check_ports>yes</check_ports>
<check_sys>yes</check_sys>
<check_trojans>yes</check_trojans>
<check_unixaudit>yes</check_unixaudit>
</rootcheck>
Step 4: Configure Log Analysis Rules
<!-- Custom rules in /var/ossec/etc/rules/local_rules.xml -->
<group name="local,syscheck,">
<!-- Alert on critical binary modifications -->
<rule id="100001" level="12">
<if_sid>550</if_sid>
<match>/usr/bin/|/usr/sbin/|/bin/|/sbin/</match>
<description>Critical system binary modified: $(file)</description>
<group>syscheck,pci_dss_11.5,</group>
</rule>
<!-- Alert on new executable in temp directories -->
<rule id="100002" level="10">
<if_sid>554</if_sid>
<match>/tmp/|/var/tmp/</match>
<description>New file created in temp directory: $(file)</description>
<group>syscheck,malware,</group>
</rule>
<!-- Alert on SSH configuration changes -->
<rule id="100003" level="10">
<if_sid>550</if_sid>
<match>/etc/ssh/sshd_config</match>
<description>SSH configuration modified</description>
<group>syscheck,authentication,</group>
</rule>
</group>
Step 5: Configure Active Response
<!-- Auto-block IP after repeated authentication failures -->
<active-response>
<command>firewall-drop</command>
<location>local</location>
<rules_id>5712</rules_id>
<timeout>600</timeout>
</active-response>
<!-- Disable account after brute force detection -->
<active-response>
<disabled>no</disabled>
<command>disable-account</command>
<location>local</location>
<rules_id>100100</rules_id>
<timeout>3600</timeout>
</active-response>
Step 6: Integrate with SIEM
# Wazuh to Splunk via Filebeat
# Edit /etc/filebeat/filebeat.yml:
filebeat.inputs:
- type: log
paths:
- /var/ossec/logs/alerts/alerts.json
json.keys_under_root: true
output.elasticsearch:
hosts: ["https://splunk-hec:8088"]
# Wazuh to Elastic via direct integration
# Wazuh indexer feeds directly into OpenSearch/Elasticsearch
# Dashboard: https://wazuh-dashboard:5601
Key Concepts
| Term | Definition |
|---|---|
| HIDS | Host-based Intrusion Detection System; monitors individual endpoints for malicious activity |
| FIM | File Integrity Monitoring; detects unauthorized changes to files by comparing cryptographic hashes |
| Syscheck | Wazuh/OSSEC module for file integrity monitoring and registry monitoring |
| Rootcheck | Wazuh/OSSEC module for rootkit and malware detection |
| Active Response | Automated defensive action triggered by HIDS alert (IP block, account disable) |
| CDB List | Constant Database list used for custom lookups in Wazuh rules |
Tools & Systems
- Wazuh: Open-source HIDS platform (fork of OSSEC) with manager, agent, and dashboard
- OSSEC: Original open-source HIDS (predecessor to Wazuh)
- AIDE (Advanced Intrusion Detection Environment): Standalone file integrity checker for Linux
- Tripwire: Commercial file integrity monitoring solution
- Samhain: Open-source HIDS focused on file integrity and log monitoring
Common Pitfalls
- Monitoring too many directories: FIM on entire filesystems generates excessive alerts. Focus on critical system binaries, configuration files, and web roots.
- Not excluding noisy files: Frequently changing files (logs, temp, caches) generate false positive FIM alerts. Maintain exclusion lists.
- Ignoring baseline establishment: First FIM scan creates a baseline. Changes detected before baseline stabilization are noise, not threats. Allow 48 hours for baseline.
- Active response without testing: Auto-blocking IPs or disabling accounts can cause outages. Test active response rules in a non-production environment first.
- Agent enrollment failures: Agents must successfully enroll with the manager before monitoring begins. Verify firewall rules allow port 1514 and 1515 traffic.