Extending OSSIM: Plugins, Integrations, and Custom Correlation RulesAlienVault OSSIM (Open Source Security Information Management) is a powerful open-source security information and event management (SIEM) platform that combines multiple security controls into a single console. While its out-of-the-box capabilities provide a solid foundation for log collection, normalization, correlation, and basic incident management, many organizations require custom extensions to tailor OSSIM to their environment. This article covers practical approaches to extending OSSIM through plugins, integrations, and custom correlation rules, with implementation guidance, best practices, and example use cases.
Why extend OSSIM?
OSSIM’s modular architecture was designed to accommodate diverse environments and changing threat landscapes. Extending OSSIM lets you:
- Improve detection of environment-specific threats.
- Integrate additional data sources (cloud services, custom applications, new network devices).
- Automate threat response and remediation.
- Reduce false positives by refining correlation logic.
- Enrich events with contextual data (asset inventory, vulnerability data, user identity).
OSSIM architecture overview (brief)
At a high level, OSSIM includes:
- Sensor data collection agents (e.g., syslog, SNMP, OSSEC, Suricata/Snort IDS).
- OSSIM server components: data normalization and storage, event correlation engine (SIEM), web GUI, and database (MySQL/alienvault-db).
- Plugin framework for parsing/normalizing incoming data.
- Correlation directives (rules) that generate alarms when specific conditions are met. Understanding the flow — from raw log to normalized event to correlated alarm — is essential when designing extensions.
Plugins: adding new log sources and parsers
Plugins in OSSIM are used to parse and normalize incoming logs so the correlation engine can interpret them consistently.
What a plugin does:
- Recognizes log source and format.
- Maps log fields to OSSIM event fields (protocol, src/dst IP, user, action, etc.).
- Assigns a plugin ID and priority for processing.
Where to add or edit plugins:
- Plugin files are stored under /etc/ossim or /usr/share/ossim/plugins (path may vary by version); XML files define parsing rules.
- Use ossim-reconfig or appropriate service restarts after changes.
Steps to create a basic plugin:
- Identify the log format from the device/application.
- Define the plugin XML with a unique ID, name, and regex patterns to extract fields.
- Map extracted fields to OSSIM standard fields (src_ip, dst_ip, port, protocol, user, msg).
- Test locally by sending sample logs to the OSSIM sensor (e.g., via logger or netcat) and confirm normalization in the event viewer.
- Iterate on regex patterns to handle log variants and edge cases.
Practical tips:
- Reuse existing plugin patterns when possible; many fields and formats recur across vendors.
- Keep plugin regexes efficient to avoid performance hits.
- Version-control custom plugins and document changes for auditability.
Integrations: connecting external systems
Integrations bring external context, enrich events, or automate actions. Common integrations include threat intelligence feeds, vulnerability scanners, cloud services, ticketing/ITSM, and SOAR/playbooks.
Examples and implementation notes:
- Threat Intelligence Feeds
- Purpose: Enrich events with reputation data (malicious IPs, domains).
- Implementation: Periodically import feeds into OSSIM’s reference tables or configure Passive DNS / external scripts to check indicators at event time.
- Benefit: Faster prioritization and blocking.
- Vulnerability Scanners (Nessus/OpenVAS)
- Purpose: Correlate asset vulnerabilities with detected alerts.
- Implementation: Import vulnerability scan results into OSSIM’s asset and vulnerability databases, or use scheduled imports/parsers. Map CVEs to asset IPs so correlation rules can consider exploitability.
- Benefit: Contextualize alerts — a detected exploit against a known vulnerable host can be escalated automatically.
- Cloud Services (AWS, Azure, GCP)
- Purpose: Collect logs from cloud-native services (CloudTrail, GuardDuty, Azure Activity Logs).
- Implementation: Use cloud-native log forwarding to a syslog endpoint or a lightweight collector (e.g., Fluentd) that forwards normalized logs to OSSIM. Create plugins for cloud-specific formats.
- Benefit: Extend visibility beyond on-prem infrastructure.
- Ticketing/ITSM (JIRA, ServiceNow)
- Purpose: Automate incident ticket creation and lifecycle tracking.
- Implementation: Use OSSIM alerts to trigger scripts or webhooks that call ticketing APIs. Include contextual fields (asset owner, severity, recommended remediation).
- Benefit: Streamlines incident response and accountability.
- SOAR / Automation Tools
- Purpose: Orchestrate multi-step responses (quarantine host, block IP, run scans).
- Implementation: Integrate via REST APIs or message queues. Trigger playbooks from OSSIM correlation rules or via an intermediary automation engine.
- Benefit: Reduces mean time to remediation.
Integration best practices:
- Authenticate and secure API calls (use keys, TLS).
- Rate-limit and cache external queries to avoid delays.
- Normalize external data into OSSIM fields for use in correlation rules.
- Monitor integration health and add failover where needed.
Custom correlation rules: design and examples
Correlation directives (rules) are the brain of OSSIM — they take normalized events and produce alarms when patterns indicate threats.
Rule-building checklist:
- Define the threat or scenario precisely (e.g., brute-force SSH, data exfiltration via DNS).
- Identify the minimal set of event types and attributes required to detect the scenario.
- Choose appropriate time windows and thresholds to balance detection vs false positives.
- Enrich with context (asset value, vulnerability) to prioritize alerts.
- Test extensively using historical logs and simulated events.
Common rule patterns:
- Brute-force login detection
- Trigger when N failed auth events for the same source IP against any internal host within T minutes, followed by a successful login.
- Consider whitelisting known scanners and legitimate automated processes.
- Example variables: N=10, T=5 minutes.
- Data exfiltration over DNS
- Detect large number of DNS TXT or unusual long DNS queries from internal hosts to external resolvers within a short window.
- Combine with unusual outbound traffic volume from the host.
- Lateral movement detection
- Sequence: Remote exploit alert (IDS) against host A → suspicious SMB authentication from host A to host B → unusual command execution logs on host B.
- Use asset tagging to prioritize when host A or B are high-value systems.
- Vulnerability exploitation
- Correlate IDS signatures that indicate exploitation attempts with a recent vulnerability scan showing the target host as vulnerable to the exploited CVE.
Example custom rule (conceptual)
- Name: “SSH brute-force then success”
- Conditions:
- Event type = SSH_AUTH_FAIL AND count by src_ip >= 15 within 10 minutes
- Followed by Event type = SSH_AUTH_SUCCESS from same src_ip to same dst_ip within 30 minutes
- Asset value of dst_ip >= medium
- Action: Generate high-priority alarm; create ticket; block src_ip via firewall integration.
Rule tuning:
- Start with broader detection, then refine using false-positive analysis.
- Use thresholds adaptive to normal traffic baselines for each asset group.
- Implement suppression and throttling to prevent alert storms.
Testing and validation
- Replay historical logs into a test OSSIM instance to validate plugins and rules.
- Use synthetic log generators or attack frameworks (in controlled lab environments) to trigger scenarios.
- Maintain a test dataset with labeled events to measure true/false positive rates after rule changes.
- Log changes (plugins, rules) in version control; document rationale and test results.
Performance and scaling considerations
- Efficient parsing: expensive regexes on high-volume feeds can bottleneck ingestion.
- Distributed collection: use multiple sensors to spread load; centralize correlation where sensible.
- Database tuning: tune MySQL settings (buffers, connection limits) as event volume grows.
- Archiving: move old raw logs to offline storage to keep the active dataset performant.
Governance, change control, and documentation
- Use a change-management process for plugin and rule updates (review, testing, approval).
- Keep a catalog of custom plugins, mappings, and correlation rules with owner and purpose.
- Periodically review rules for relevance and update thresholds according to evolving baselines.
Example real-world use case
A mid-sized company integrated Nessus scan results and a commercial threat feed into OSSIM. They created a correlation rule that matched high-severity CVE findings with IDS exploit attempts. Within weeks this produced multiple high-confidence alarms that led to prioritized patching and rapid containment — reducing dwell time on several compromises.
Final tips
- Prioritize integrations that give the most context for your critical assets.
- Invest time in accurate asset inventory and vulnerability import — it amplifies rule effectiveness.
- Automate safely: start with notifications, then gradually enable automated responses once confidence is high.
- Keep plugin and rule changes auditable and reversible.
Extending OSSIM through plugins, integrations, and custom correlation rules is a continuous process: as your environment and threats evolve, so should your SIEM. Focus on high-value data sources, test thoroughly, and tune rules to minimize noise while maximizing meaningful detections.
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