Open Source SIEM: The Best Free SIEM Tools and Systems Compared

We're the engineering team at WhyCrew, and we work inside these tools every day. This guide covers which free and open source SIEM tools are worth your time, how they work, what they really cost and when a free SIEM stops being enough.

This guide is for security leaders, SOC managers, IT teams, MSSPs and home-lab learners comparing open source SIEM tools, free SIEM options and custom-built platforms.

What Is an Open Source SIEM?#

An open source SIEM is a security information and event management system with public source code. Anyone can use it, read it and change it, and there's no license fee. Like any SIEM, it gathers security events from across your systems, standardizes them, checks them against detection rules and alerts your team when something looks wrong.

Most open source SIEMs aren't a single program. They're platforms made from several open source projects. For example, OpenSearch handles storage and search, Zeek and Suricata watch the network, and Sigma provides detection rules.

Open Source vs Source-Available vs Free SIEM#

SIEM software falls into three groups.

All three can be the right choice, as long as you check the license before you build a business on it. If SIEM itself is new to you, start with what a SIEM does and how it works, then come back to compare tools.

What Are SIEM Tools and SIEM Systems?#

SIEM tools are security platforms that combine log management with real-time event correlation. They help a team spot threats, look into incidents and prove compliance, all from one place. A "SIEM system" means the whole setup: the software, plus the servers, storage, connections and rules that make it useful.

Core functions of a SIEM

Open source or paid, every SIEM does four main jobs.

Log aggregation: it gathers logs and events from servers, firewalls, laptops, cloud services and apps into one place you can search.

Event correlation: it links related events from different sources into one alert, so analysts see the full picture instead of scattered clues.

Compliance reporting: it keeps security records for as long as the rules require and creates evidence for standards like PCI DSS, HIPAA, ISO 27001 and GDPR.

Response integration: it sends alerts to ticket systems and SOAR platforms, which run predefined playbooks to stop threats faster.

Types of SIEM tools

There are four common types. Free and self-hosted SIEMs, like Wazuh or Security Onion, run on your own Linux servers or cloud accounts with no license fee. Commercial on-premises SIEMs, like Splunk Enterprise Security or IBM QRadar, are paid software you run yourself. Cloud-native SIEMs, like Microsoft Sentinel or Google Security Operations, are paid services billed by data or usage. Managed SIEM services add a provider's analysts on top of any of these.

There's also a fifth option in between: a custom-built, owned SIEM platform. It's built around your own log sources, clients and compliance needs, on open source tools like OpenSearch or Wazuh. You pay for it once instead of renting it.

How Does an Open Source SIEM Work?#

An open source SIEM works in five stages. It collects logs, puts them in one format, stores them, checks them against rules and sends alerts to analysts. If one stage is weak, every stage after it suffers.

Collection. Agents on servers and laptops, syslog from network devices, and API connections to cloud services bring events in.

Normalization and enrichment. Every product writes logs in its own way. So the SIEM maps fields like user, host and IP address into one shared format. Open formats such as OCSF (Open Cybersecurity Schema Framework) and ECS (Elastic Common Schema) save you from building your own. Enrichment then adds context, like who owns a server or whether an IP address is known to be malicious.

Storage and indexing. Events go into a search engine like OpenSearch, or a fast analytics database like ClickHouse.

Detection and correlation. Simple rules flag single risky events. Correlation rules link related events from different sources. This is the step that turns a pile of logs into a useful security tool.

Investigation and response. Analysts look at the evidence and work out what happened. Many teams let automated SOAR playbooks handle the repeat steps. They keep a human sign-off for big actions, like disabling an account.

Example: three ordinary events, one alert

Let's say an employee's account has a burst of failed sign-ins one evening. A few minutes later, it signs in from a device nobody has seen before. Soon after, the cloud audit log shows an admin role being added to that same account.

Each event could be innocent on its own. Together, on one account in a short time, they look a lot like an account takeover. The SIEM pulls the evidence from three systems into one alert. An analyst then checks it before anyone acts.

The Best Open Source SIEM Tools#

The seven free and open source SIEM tools worth a look are Wazuh, Security Onion, OpenSearch Security Analytics, UTMStack, Elastic Security, Graylog Open and OSSEC. They're listed from the most complete platform to the most specialized.

How we evaluated these tools: We checked each tool's license and main features against its official documentation. We made sure it's still actively maintained. Then we looked at how much of the SIEM job it covers on its own, from collecting logs to compliance reports. We also weighed how easy it is to set up, how big its community is, and our own experience building platforms on OpenSearch and Wazuh.

Retired or limited tools you'll still see in older lists

Some "best open source SIEM" articles haven't been updated in years. They still recommend projects that are no longer maintained or are too limited for real use.

AlienVault OSSIM was once the go-to free SIEM. It was discontinued after AlienVault became part of AT&T Cybersecurity, now LevelBlue, and it no longer gets updates. Apache Metron was retired by the Apache Software Foundation in 2020 and archived in 2021. Prelude OSS is the limited free edition of a paid SIEM and is mainly useful for testing.

Splunk Free shows up on many of these lists too, but it isn't open source. It's a free license with a 500 MB daily data limit, no alerts and no user authentication. It's fine for learning, but not for production monitoring.

Open Source SIEM Tools Compared#

Licenses and features can change, so check each project's documentation before you decide.

All seven are general-purpose products. None of them was built around your clients, your data rules or your compliance needs, so you end up changing how you work to fit the tool.

Wazuh vs Security Onion: Which One Should You Choose?#

In short: Wazuh is better for monitoring servers and laptops, Security Onion is better for network visibility and threat hunting, and many teams run both.

Wazuh and Security Onion are both free SIEM platforms, but they do different jobs.

Wazuh puts an agent on every server and laptop. That lets it see programs, file changes, unsafe software, bad settings and sign-ins. It also scales to thousands of devices.

Security Onion sits on the network instead. Using Zeek and Suricata, it records connections and suspicious traffic, and it can save full copies of network traffic for deep investigations. It catches things that never touch a monitored device, like an unknown device talking to a malicious server. The downside is that it needs more hardware and more network skills.

If you run both, plan how to bring their alerts into one place, because switching between two dashboards adds work.

The Best Free SIEM for Your Situation#

The best free SIEM depends on who will run it and what you need to watch. For home labs, start with Wazuh. Small businesses usually do best with Wazuh or UTMStack, network-heavy and OT sites with Security Onion, and MSSPs with a multi-tenant platform built on OpenSearch or Wazuh.

Best free SIEM for a home lab

One virtual machine is enough to run Wazuh's server, indexer and dashboard for a few agents, and the documentation is very detailed. Want to learn network monitoring too? Add Security Onion on a second machine, connected to a network tap or a mirrored switch port. Building both is a practical way to prepare for a SOC analyst job.

Best SIEM for small businesses

If you have some in-house IT skills, Wazuh or UTMStack give you the most coverage for the least effort. The bigger question is who will read the alerts. If nobody has time, a managed service will likely protect you better than a free tool that nobody checks.

Best open source SIEM for MSSPs

MSSPs need things a single company doesn't: fully separate data for every client, per-client retention rules, and branded portals and reports. Open source tools, especially OpenSearch and Wazuh, give you the building blocks. But turning them into a reliable multi-tenant SIEM platform takes serious engineering.

Best SIEM for operational technology (OT) environments

Factories and other critical sites need monitoring that never disturbs delicate machines. Security Onion is a strong free choice here. Its built-in Zeek sensor can read common industrial protocols like Modbus and DNP3 without touching the devices. Wazuh can then cover the Windows and Linux computers on site, like engineering workstations and control screens (HMIs). Larger sites often add a dedicated paid OT security product that sends its alerts into the SIEM.

Building a Modular Open Source SOC Stack#

A SIEM is the center of a security operations setup, but it isn't the whole setup. Many teams build a modular open source stack. They pair a main SIEM like Wazuh or OpenSearch with a specialist tool for each job.

Log routing: Graylog, Fluent Bit or Vector collect, sort and route large amounts of logs before they reach the SIEM. This helps keep storage costs down.

Dashboards: Grafana is popular for custom dashboards and for tracking alert trends.

Case management and SOAR: TheHive or DFIR-IRIS track investigations, and Shuffle offers drag-and-drop automation.

Threat intelligence: MISP and OpenCTI collect and share signs of known attacks, then feed them into the SIEM.

Detection content: Sigma provides ready-made detection rules, written by the community, that work with most SIEMs.

This approach is flexible and avoids lock-in, but every extra tool is one more thing to update, secure and connect, and the glue between them is often scripts only one engineer understands.

Top Commercial SIEM Tools and How They Compare to Open Source#

Most "top SIEM tools" lists are full of paid platforms like Splunk Enterprise Security, Microsoft Sentinel, Google Security Operations, IBM QRadar and Exabeam. The details vary, but the business model is the same. You rent the platform, and the bill usually grows with the amount of data you send and keep. That's the main difference between paid and open source SIEM tools.

Some MSSPs and regulated teams take a third path: they leave a rented platform for one they own, using a zero-downtime SIEM migration so coverage never drops during the switch.

Pros and Cons of Free SIEM Systems#

The biggest benefit is cost control: with open source, more data means more hardware, not more license fees. You also get full control over where your data lives and who can see it, which matters for data residency rules. And when a detection acts strangely, your engineers can read the code themselves.

The main downside is the work of running it. You can install a SIEM in a day, but turning it into a reliable service with steady data, tuned rules, backups and an incident plan takes months of skilled work. Unless you pay a partner, there's no vendor to call when it breaks. So the question is whether you have, or can buy, the skills to run it well.

How Much Does an Open Source SIEM Really Cost?#

You pay nothing for the license itself. The real cost comes from four places: storage, compute, engineering time and the analysts who watch the alerts. Storage is the easiest to estimate. Engineering and people usually cost the most.

A simple storage estimate

Imagine a setup that creates 1,000 events per second on average, and each event is about 800 bytes. That's about 69 GB of raw data a day (1,000 × 800 bytes × 86,400 seconds). Over 90 days, it adds up to roughly 6.2 TB. Over a year, it's about 25 TB.

Those are raw numbers. Indexing adds extra space. One replica doubles the total, and backups add more. Compression brings the total back down a bit.

The costs a quote never shows

Computing power grows with every rule, data source and long search during an incident. Engineering time is the cost teams forget most: building connections, fixing parsers when vendors change log formats, testing updates and keeping rules current. And a SIEM nobody watches at night only protects you during the day.

A fair comparison looks at three to five years and weighs licensed SIEM pricing against the full cost of a platform you own.

Self-Hosted SIEM Architecture: Single Server or Distributed?#

Most open source SIEM systems are self-hosted on Linux, either on your own hardware or in your own cloud account. How you set up the parts decides how fast, reliable and costly the platform will be.

A single-node deployment runs collection, detection, storage and the dashboard on one server. It's quick to set up and works well for a home lab, a test or a small business. The catch is that everything shares one machine. A heavy search can slow down incoming data, and if that server fails, monitoring stops.

A distributed deployment splits the parts across several machines. Several collectors receive data, so one failure doesn't lose events. A queue like Apache Kafka absorbs sudden spikes, such as the jump in logs during an attack. A storage cluster keeps copies of data on more than one server. Detection and analyst tools get their own resources too. But every extra part needs updates, monitoring and access control, so pick the simplest design that meets your needs.

Hardware requirements: a Wazuh example

Wazuh's official quickstart runs the server, indexer and dashboard together on one 64-bit Linux machine. Wazuh says this is usually enough for up to 100 devices with 90 days of alert data.

For more than 100 devices, Wazuh recommends a distributed setup. Keep in mind these numbers are for alerts from typical devices. Busy sources like firewalls or network sensors need much more storage.

Hot, warm and cold storage

Most SIEM data is searched a lot for a few weeks, then hardly at all. Tiered storage is built around that. Hot storage keeps recent events on fast disks for investigations. Warm storage keeps older data on cheaper hardware. Cold or archive storage keeps long-term records at the lowest cost for audits. Getting these tiers right is usually the best way to cut storage costs.

How to Choose and Deploy a Free SIEM#

Don't choose a SIEM by its feature list. Judge it by how it handles your data, your volumes and your team.

Questions to answer before you choose

Ask which log sources you need to cover (Windows, Linux, firewalls, Microsoft 365, AWS) and how well the tool reads them. Check the quality of its built-in detection rules and whether it supports Sigma and MITRE ATT&CK. Find out how it grows from one server to many, what it connects to, how active the project is, and whether it produces the compliance reports you need.

Deploying it, step by step

1. 01Decide what you need to detect. List the few threats that would hurt your business most, like stolen passwords or unapproved admin changes. These decide which logs matter.
2. 02Start with a few important sources. That's usually your sign-in system (identity provider), admin activity on key servers, firewall or VPN logs, and your main cloud account. Collecting everything on day one just adds cost and noise.
3. 03Measure your volume and size the platform. Run a week of sample data. Measure events per second and event size. Then size your storage and servers from real numbers, not guesses.
4. 04Build a test setup and check the data. Before you write any rules, make sure events arrive on time, timestamps are right, and fields like user and host are read correctly.
5. 05Add and test detections. Trigger each rule on purpose, in a safe way, and check that the alert shows the right evidence. Then filter out what's normal for your systems.
6. 06Decide who responds. Agree who gets urgent alerts at night and on weekends. Decide how rule changes are approved and what happens when a log source goes quiet. Then grow step by step, repeating steps 2 to 5.

Common mistakes to avoid

Default rules know nothing about your environment, so don't trust them as they come. Check data quality early, because bad fields make correlation fail silently. Plan for upkeep too, since collectors break and certificates expire. The biggest mistake is having no clear owner for alerts. A SIEM can raise exactly the right alarm and still fail if nobody acts on it.

Detection Engineering: Where Free SIEMs Succeed or Fail#

Many teams collect huge amounts of logs and still miss attacks. The difference is detection engineering: the ongoing work of writing, testing and updating the rules that decide what becomes an alert.

Sigma rules give you a head start, with thousands of community rules most open source SIEMs can use. But a rule only fires if your data matches it, so check every rule's fields and test it. MITRE ATT&CK shows which attacker techniques you cover. In October 2025, version 18 replaced its old Data Sources with Detection Strategies and Analytics, so older coverage reports may need an update. Treat coverage as a map, not a score.

The best proof is to safely attack yourself. Breach and attack simulation (BAS) and purple-team exercises replay known attacker techniques and show which ones your SIEM catches. Track results like Mean Time to Detect (MTTD) and Mean Time to Respond (MTTR), and keep false alarms down, since constant noise is a major cause of SOC analyst burnout.

Security and Compliance#

Is an open source SIEM secure?

You can run one securely, but remember that it's one of the most valuable targets on your network. It holds usernames, computer names, internal IP addresses and a record of every admin action. That's a gift to any attacker who gets in.

Treat it like the critical system it is. Keep its admin screens on a separate, locked-down network, and require multi-factor authentication for every admin. Encrypt traffic between agents and servers, and install updates on a regular schedule. Keep audit evidence on write-once (immutable) storage, so nobody can quietly rewrite history. Finally, run a separate health check that warns you when collectors stop sending data. A SIEM can't warn you about its own outage.

Can an open source SIEM support compliance?

Yes. It can collect, protect and keep the security evidence that standards like ISO 27001, PCI DSS and HIPAA expect. It can also support rules such as GDPR, NIS2 and DORA. Auditors care about your controls, not whether you paid for the software.

What a SIEM can't do is make you compliant by itself. It covers the logging and monitoring part. It shows that the right events are collected, kept long enough, protected from tampering, reviewed by someone and turned into incident reports on time. In regulated industries, also check timestamps, who can see the logs and where the data is stored.

This is often where general-purpose tools fall short, because each regulator wants evidence in its own format. WhyCrew builds those needs into the platform from day one. That includes NIS2 and DORA compliance automation, with incident-reporting workflows and EU data residency. It also includes SIEM platforms designed for NCA ECC and SAMA CSF compliance in Saudi Arabia. Rules still vary by country and industry, so check your obligations with a qualified compliance advisor.

AI and the Future of SIEM#

The next big change in SIEM is about understanding logs faster, not collecting more of them.

AI-assisted triage: AI models can sum up alerts, find related events and suggest next steps, which saves Tier-1 analysts a lot of time. But AI can be confidently wrong, and many cloud AI assistants send your data to an outside company. That's one reason teams consider an AI SOC model and choose to run the AI models inside their own environment, so alert data never leaves their network.

UEBA: User and entity behavior analytics learns what normal looks like for each user and device, then flags unusual activity like sign-ins at odd hours. It's mostly a paid-platform feature today, but teams can build similar checks on OpenSearch with its anomaly detection plugin.

Open standards: OCSF, Sigma and MITRE ATT&CK make it easier to move data and rules between tools, which means less lock-in. When you decide what to automate, a simple reversibility test for AI SOC automation helps: easy-to-undo actions can run on their own, while bigger ones keep a human in the loop.

When a Free Open Source SIEM Is No Longer Enough#

There's no exact moment when a free SIEM stops working. You notice it as growing friction. Searches slow down as data grows. Engineers spend more time fixing connections than improving detections. Or an auditor asks how your evidence is protected, and nobody has a clear answer.

MSSPs usually hit this point first, as their client count grows. Regulated companies hit it when auditors expect fast incident reports under NIS2 and DORA or monitoring evidence under NCA ECC and SAMA CSF.

Reaching these limits doesn't mean throwing open source away. The smarter move is to keep the open source parts that work and build the missing layers around them. The choice comes down to the open source vs custom-built SIEM trade-off. Who owns the roadmap? Who does the upkeep? And which model handles multi-tenancy and compliance best for your business?

How WhyCrew fits in

We're not a free SIEM vendor, and we're not a managed security provider. WhyCrew is an engineering team, based in Canada, that designs and builds custom SIEM, SOAR and SOC platforms for MSSPs and regulated companies. When the build is done, we hand it over.

We build on proven open source tools like OpenSearch, Elasticsearch, ClickHouse and Wazuh wherever they fit. Then we build what open source doesn't cover. That includes white-label, multi-tenant SOC platforms for MSSPs, tuned data pipelines, detection rules written for your systems, compliance reports and private AI agents for SOC automation that keep alert data inside your network.

Every project is a fixed-price build, not a subscription. You pay once for the platform. At handover, you get the source code, the infrastructure, the documentation and hands-on training. That's how one growing MSSP moved from renting a SIEM to an owned security platform without any gap in coverage.

Frequently Asked Questions About Open Source SIEM#