What are the best free and open-source SIEM tools in 2025?

Free and open-source SIEM tools have become essential for organizations seeking enterprise-grade threat detection and log correlation without high costs. In 2025, top tools like OSSIM, ELK Stack, Wazuh, and MozDef offer advanced features including real-time alerting, log aggregation, compliance monitoring, and threat intelligence integrations. These solutions provide flexibility for teams of all sizes—ranging from small security labs to enterprise SOCs—looking to monitor and respond to security incidents effectively.

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Security Information & Event Management (SIEM) combines real‑time log collection, correlation, alerting, and reporting to help organizations spot threats fast. While commercial SIEM suites can cost six figures, open‑source SIEM tools deliver many of the same features without the hefty license fees—perfect for small teams, startups, labs, or anyone who wants to test-drive SIEM on a budget.

Below is a curated list of the 10 best open‑source (or free‑tier) SIEM solutions you can deploy in 2025. For each tool, you’ll find a quick feature snapshot, ideal use cases, and a few pros & cons to guide your selection.

Why Choose an Open‑Source SIEM?

Cost‑effective: No per‑ingest or per‑node license fees.
Flexibility: Customize source code, parsers, and dashboards.
Community Support: Active forums, GitHub issues, and plug‑ins.
Learning Value: Great for hands‑on practice and skills growth.

OSSIM – AlienVault’s Hybrid SIEM

All‑in‑one threat detection with built‑in asset discovery.

Key Features	Details
Server / Agent Model	Sensors collect logs; central server correlates events.
Integrated IDS	Includes Suricata and Snort plugins.
Threat Intelligence	Built‑in OTX (Open Threat Exchange) feeds.

Pros: Rich plug‑in ecosystem, unified dashboard, easy ISO installer.
Cons: Resource‑heavy; some features reserved for paid USM Anywhere.

Sagan – Real‑Time Correlation Engine

Fast, Snort‑like rule syntax for log correlation.

Works with: Snorby, EveBox for GUI dashboards.
Speed: Multithreaded, written in C for high performance.
Output: Can generate Snort‑compatible alerts for downstream tools.

Best For: SOCs needing ultra‑fast correlation without extra bloat.

Splunk Free – 500 MB/Day Indexing Tier

Industry‑grade analytics—at a hobbyist data volume.

Limit: ~500 MB/day ingest; no Enterprise features (clustering, LDAP).
Real‑Time Alerts: Same powerful SPL searches as paid Splunk.
App Store: Thousands of community add‑ons for dashboards.

Trade‑off: Great to learn Splunk; not scalable for mid‑size log loads.

Snort – Packet‑Level Network IDS (SIEM Add‑on)

Still the gold‑standard for deep packet inspection.

Detection Engine: Signature & protocol‑aware, inline or passive.
Community Rules: Regularly updated threat signatures.
SIEM Hook: Send Snort alerts to any syslog‑based SIEM (e.g., ELK, Wazuh).

Note: Snort focuses on network traffic; pair it with a log engine for full SIEM.

Elasticsearch – DIY Log Search & Scan

A search engine at heart, but perfect for high‑volume security data.

Horizontal Scale: Shards + replicas for petabyte storage.
Query DSL: Powerful search and aggregation for anomaly hunting.
Plug‑ins: Add Alerting, ML, or Security modules (open‑source or basic tier).

DIY Factor: Requires Logstash/Beats for parsing, Kibana for visuals.

MozDef (Mozilla Defense Platform)

Microservices‑driven SIEM built for cloud scale.

Stack: Python, Elastic, RabbitMQ, and Docker containers.
Automation: Playbooks trigger response actions (e.g., block IP).
Integrations: Telemetry from AWS, GCP, Azure, and on‑prem logs.

Ideal For: Teams comfortable with DevOps pipelines and container orchestration.

ELK Stack (Elasticsearch + Logstash + Kibana + Beats)

The classic open‑source SIEM foundation.

Component	Role
Beats / Logstash	Collect & parse logs
Elasticsearch	Store & index events
Kibana	Visualize & alert

SIEM App: Built‑in Kibana dashboards for detections and hosts.
X‑Pack Basic: Free features include Alerting and Security.

Pros: Massive community, countless tutorials.
Cons: Resource demands grow quickly; tuning needed for high EPS.

Wazuh – Open‑Source XDR & Compliance Suite

Host‑based threat detection plus centralized SIEM.

Modules: File Integrity Monitoring, Rootkit detection, Cloud‑trail parsing.
Elastic Integration: Ships with ready‑made Kibana dashboards.
Compliance: CIS, PCI‑DSS, NIST templates out of the box.

Standout: Strong agent‑based detection on Linux/Windows/Mac alongside log SIEM.

Apache Metron

Big‑data SIEM originally from the Hadoop ecosystem.

Data Lake: Kafka → Storm → HBase pipeline for massive ingest.
Parser Flexibility: Enriches events with geoIP, threat intel.
SOC Dashboard: Real‑time triage interface built on Kibana.

Caveat: Hadoop/HBase skills required; slower community activity than ELK.

Quadrant/Sagan – Real‑Time Log Correlation + Visual Console

A forked and enhanced Sagan stack backed by Quadrant Security.

UI: Web‑based console for live alerts and rule management.
Machine Learning (beta): Statistical anomaly scoring.
Cloud Support: Agents for AWS/GCP log pipelines.

Good Fit: MSPs and MSSPs needing a multi‑tenant, real‑time SIEM.

Quick Comparison Table

SIEM Tool	Real‑Time Correlation	Built‑In GUI	Ease of Setup	Best For
OSSIM	✔	✔	Medium	All‑in‑one starter SIEM
Sagan	✔ (fast)	(via Snorby)	Medium	High‑speed parsing
Splunk Free	✔	✔	Easy	Learning Splunk
Snort	✔ (network)	✖	Medium	IDS add‑on
Elasticsearch	✖ (needs alerting)	Kibana	DIY	Massive log volumes
MozDef	✔	✔	Advanced	DevOps SOCs
ELK Stack	✔	✔	Medium	Custom SIEM builds
Wazuh	✔	✔	Easy	Endpoint + SIEM combo
Apache Metron	✔	✔	Advanced	Big‑data SOC
Quadrant/Sagan	✔	✔	Medium	MSSPs / MSPs

Choosing the Right Open‑Source SIEM

Define log volume & retention – High EPS? Choose horizontally scalable stacks (ELK, Metron).
Assess skill requirements – Comfort with Linux, Docker, or Hadoop?
Check community & updates – Active projects like Wazuh and ELK evolve quickly.
Consider compliance needs – PCI or HIPAA? Wazuh and OSSIM ship with templates.
Plan for alert fatigue – Start with baseline rules; tune over time.

Conclusion

Open‑source SIEM tools have matured to rival commercial solutions, offering real‑time detection, scalable storage, and customizable dashboards—often at zero licensing cost. Whether you’re a small business protecting critical assets or a large enterprise building a full SOC, the ten tools listed above provide a solid starting point. Test‑drive in a lab, benchmark ingest capacity, and tailor rulesets to your environment for the best results.

Happy hunting, and stay secure!

FAQs

What is a SIEM tool?

A SIEM (Security Information and Event Management) tool collects, correlates, and analyzes security data in real time for threat detection and incident response.

What are the best free SIEM tools in 2025?

Top tools include OSSIM, Wazuh, ELK Stack, MozDef, and Splunk Free.

Is Splunk SIEM free?

Yes, Splunk offers a free version with a 500MB/day limit on data indexing.

What is OSSIM used for?

OSSIM is a hybrid open-source SIEM by AlienVault that combines IDS, threat detection, and asset discovery.

Is ELK Stack a SIEM?

Yes, when configured with Logstash and Kibana, ELK can function as a powerful open-source SIEM.

What is Wazuh?

Wazuh is an open-source XDR and SIEM platform that includes host-based intrusion detection, file integrity monitoring, and log analysis.

What makes a good SIEM tool?

Features like real-time correlation, threat intelligence integration, customizable alerts, and scalability make a SIEM effective.

Can I use Snort as a SIEM?

Snort itself is an IDS, but its alerts can feed into SIEM tools for centralized monitoring.

What is Sagan?

Sagan is a fast, real-time log correlation engine that works well with other open-source SIEM dashboards.

What is MozDef?

MozDef is Mozilla’s microservices-based open-source SIEM with automation and cloud integration features.

Are open-source SIEM tools secure?

Yes, if properly configured and regularly updated, open-source SIEMs can be very secure.

Which SIEM supports threat intelligence feeds?

OSSIM and ELK (via plug-ins) support threat intelligence feeds like OTX or STIX/TAXII.

What is Apache Metron used for?

Apache Metron is a big data SIEM tool for ingesting and correlating logs at scale using Hadoop components.

Can SIEM tools detect ransomware?

Yes, many SIEM tools detect indicators of compromise like unusual file access, lateral movement, and data exfiltration.

What’s the difference between SIEM and IDS?

IDS detects intrusions; SIEM correlates multiple data sources to identify broader threats and compliance violations.

Does Wazuh have a dashboard?

Yes, Wazuh integrates with Kibana and comes with pre-built dashboards.

Is ELK Stack free to use?

Yes, the core ELK Stack is open-source and free to use, though advanced features may require a license.

How do I set up an open-source SIEM?

Most SIEM tools have install guides; some like OSSIM provide ISO images, while others like ELK require manual setup.

Can I use SIEM for compliance?

Yes, tools like Wazuh and OSSIM come with built-in compliance templates (e.g., PCI-DSS, HIPAA).

What are the pros of open-source SIEM?

Cost savings, customization, transparency, and community support are major advantages.

What are the cons of open-source SIEM?

They often require manual configuration, deeper technical skills, and limited vendor support.

What’s a good SIEM for small businesses?

Wazuh and OSSIM are excellent for small to mid-sized companies with security monitoring needs.

Does SIEM monitor cloud infrastructure?

Yes, with proper integration, SIEMs can monitor AWS, Azure, and GCP logs and services.

What programming languages are used in SIEM tools?

Common languages include Python (MozDef), C (Sagan), Java (Logstash), and Bash (scripts).

How much data can open-source SIEMs handle?

Tools like ELK Stack and Apache Metron are capable of handling petabytes of data.

Is SIEM required for a SOC?

Yes, SIEM is foundational for most Security Operations Centers (SOCs).

What is SIEM correlation?

Correlation links events across logs to identify complex attack patterns or rule violations.

How to reduce SIEM false positives?

Tune detection rules, whitelist known behavior, and use threat intelligence for enrichment.

Can SIEM be automated?

Yes, many modern SIEMs support automation via scripts, playbooks, and integrations (e.g., MozDef).

Is SIEM part of XDR?

Yes, many XDR (Extended Detection & Response) systems include SIEM capabilities for log-based detection.

What is the future of open-source SIEM?

Future developments include AI integration, scalable cloud-native deployments, and better UI/UX for analysts.