UTMStack

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Best Open Source SIEM Tools in 2026: 7 Compared, Licenses Checked

By UTMStack Team

An open source SIEM costs nothing to download. Running one well is another matter: someone has to size the servers, connect the log sources, tune the detections and produce the reports an auditor will accept. The tools below all clear the first bar. They differ a lot on the second, and on a question most comparison lists skip: what the license actually lets you do.

We build UTMStack, so read our ranking with that in mind. For every other tool we have tried to describe what it does well, linked its license file, and said plainly where it is the better choice.

The short answer, by situation:

  • Best all-in-one open source SIEM: UTMStack. Correlation, alerts, incidents, automated response and compliance reports in one install.
  • Best for endpoint-centric monitoring: Wazuh, for its agent’s file integrity, configuration and vulnerability checks.
  • Best for network threat hunting: Security Onion, if you are not reselling it as a service.
  • Best if you want to build your own: OpenSearch, or the Elastic Stack if its license works for you.
  • Best for log management first: Graylog Open.
  • Best network sensors to feed any SIEM: Suricata, Snort and Zeek.

First: which of these are actually open source?

“Open source SIEM” is what people search for, but in 2026 the tools that show up cover three different legal situations: open source licenses approved by the Open Source Initiative (OSI), source-available licenses that look similar but restrict commercial use, and free tiers of commercial products. We read each project’s own license file in October 2026 rather than trusting a comparison chart.

ToolLicenseOSI open source?What to watch for
UTMStackAGPL-3.0YesModify it and offer it over a network, and you share your changes.
WazuhGPLv2YesChanges you distribute ship with their source.
Security OnionElastic License 2.0NoNo hosted or managed service for others.
OpenSearchApache 2.0YesNone.
Elastic StackDownloads: Elastic License 2.0. Source: also AGPL-3.0 or SSPLSource yes, downloads noThe 2024 AGPL option covers source code only.
Graylog OpenSSPLNoOffer it as a service and you publish your whole stack’s source.
Suricata / Snort / ZeekGPLv2 / GPLv2 / BSDYes (engines)Snort’s best rules are paid (see below).

The two rows that catch people are Security Onion and Elastic. Security Onion is often suggested as a free base for a managed security service, and its license is the one that rules that out. Elastic’s 2024 change is real, but narrower than the headlines: build from source and you can choose the AGPL; download Elasticsearch the normal way and you are on the Elastic License.

1. UTMStack

The all-in-one platform

UTMStack puts the whole security operations workflow in one product: log collection, real-time correlation, alerts, incidents, automated response, threat intelligence and compliance reporting. One installer sets everything up on a dedicated Ubuntu 24.04 or Red Hat compatible server, usually in 15 to 30 minutes, so there is no stack of separate components to wire together.

What sets it apart is where the work happens. UTMStack checks every log against its 600+ detection rules, mapped to MITRE ATT&CK, as the log arrives, before it is stored. Related events become one alert instead of a pile of near-duplicates, and alerts can trigger response flows that block an address, disable an account or isolate a machine. For compliance, it evaluates controls such as multi-factor authentication, disk encryption and audit logging from your own logs, and keeps the evidence that decided each one. See how compliance works.

  • Pros: the most complete feature set on this list in a single install; genuine AGPL-3.0 license; built-in response automation (SOAR) and incident handling; searches months of logs in plain text or SQL; listed in CISA’s catalog of free cybersecurity tools.
  • Cons: a smaller community than Wazuh or Elastic, so fewer third-party guides. The free Community edition includes two compliance frameworks (CMMC 2.0 and GDPR); the other seven, multi-tenancy for service providers, the SOC-AI assistant, the macOS agent and single sign-on are in the paid Enterprise edition. It wants a dedicated machine: the smallest recommended server has 4 cores and 16 GB of memory.

Best for: teams that want a working SIEM with response and audit evidence without building one from parts. Install the free edition, or try the live demo with sample data first.

2. Wazuh

The endpoint-first all-rounder

Wazuh began as a fork of OSSEC and grew into a full platform: an agent on each endpoint, a server that decodes and correlates events, and an indexer and dashboard (both forks of OpenSearch) behind it. When people call Wazuh heavy, they usually mean the indexer, not the agent.

The agent does a lot on its own: file integrity monitoring, configuration checks against CIS benchmarks, vulnerability detection by matching installed software against CVE feeds, and active response scripts that can block an address or stop a process. It runs on Windows, Linux, macOS and older Unix systems, and its rules map to frameworks such as PCI DSS, HIPAA and NIST 800-53.

  • Pros: strong endpoint coverage out of the box; a large community; GPLv2 throughout; compliance mappings an auditor can follow.
  • Cons: the indexer needs memory and fast disks as log volume grows. Separating customers for a managed service is something you build yourself, and response beyond active-response scripts means adding other tools.

Best for: organizations whose priority is what happens on their servers and workstations.

If you only need the host-based part, OSSEC, the GPLv2 project Wazuh grew from, is still maintained and runs on very little. It sends alerts rather than keeping searchable logs, though, so it is not a SIEM on its own.

3. Security Onion

The network defender’s distribution

Security Onion is a complete Linux distribution rather than an application: Suricata for signature detection, Zeek for protocol metadata, full packet capture, and Elastic underneath, already wired together and tuned. Its analyst tools for alerts, hunting and cases are why people choose it over assembling the same parts themselves. It can also import a packet capture or Windows event log file for offline analysis, which makes it handy for one-off investigations.

  • Pros: the best network visibility on this list; the pieces work together from day one.
  • Cons: a steep learning curve and serious hardware needs. Security Onion ships under the Elastic License 2.0, which is source-available and forbids offering it to others as a hosted or managed service. Read it before you build a business on it.

Best for: internal security teams who hunt on network traffic.

4. OpenSearch Security Analytics

The fully open fork

OpenSearch is the Apache 2.0 fork of Elasticsearch, governed since 2024 by the OpenSearch Software Foundation, part of the Linux Foundation. Its SIEM features come from the bundled Security Analytics plugin, which runs Sigma detection rules against your logs and raises findings.

  • Pros: Apache 2.0 across the whole distribution, with nothing held back for a paid tier; Sigma rules included; neutral, foundation-led governance.
  • Cons: it is a search engine with a security layer, not a finished SIEM. Detections work best with the log types it already understands, anything else needs field mapping by hand, and response automation, case handling and compliance reporting are yours to add.

Best for: teams already running OpenSearch, or Elasticsearch users who want to leave the license question behind.

5. Elastic Stack (Elastic Security)

The search-first powerhouse

Elasticsearch, Logstash and Kibana sit underneath several tools on this page. Used directly, Elastic Security adds a detection engine, a detection rules repository Elastic publishes on GitHub, and case management, with integrations for most common log sources.

On licensing, Elastic moved off Apache 2.0 in 2021 (which is why OpenSearch exists) and added the AGPL-3.0 as a source code option in 2024. Its licensing FAQ is clear that its releases continue under the Elastic License, so the build most people install is not OSI open source.

  • Pros: scales very far; excellent search; open detection rules mean you do not write every rule yourself.
  • Cons: a platform rather than a product, so turning it into a working SIEM is an engineering project. Several features a security team eventually wants sit in paid tiers, and where that line falls has moved over the years.

Best for: organizations with engineers to build and run a search-first security stack.

6. Graylog Open

Log management first

Graylog is known for fast log search and ingestion, and its free Open edition has enough to act as a basic SIEM for smaller teams: inputs for syslog and other common formats, processing pipelines, dashboards and alerting. Graylog itself calls the Open edition source-available.

  • Pros: quick to set up; fast search; no ingestion caps or time limits on the free edition.
  • Cons: correlation rules, behavioral analytics and prebuilt detection content are in the paid Graylog Security edition. The SSPL license restricts offering it as a service.

Best for: teams whose first problem is collecting and searching logs, with security detection second.

7. Suricata, Snort and Zeek

The network sensors

These are not SIEMs: they watch network traffic and produce alerts or metadata, which a SIEM then stores, correlates and acts on. Suricata and Snort match traffic against signature rules; Zeek turns traffic into detailed logs of connections, DNS lookups, certificates and more. All three feed tools like UTMStack, Wazuh and Security Onion.

  • Pros: proven engines with open licenses (GPLv2 for Suricata and Snort, BSD for Zeek); the Emerging Threats Open ruleset is free.
  • Cons: Snort’s best rules are not free: subscribers get the Talos ruleset immediately, while free registered users get it 30 days later. On their own, none of them keeps or searches history.

Best for: adding network detection to whichever SIEM you choose.

People type them interchangeably, but they lead to different places.

  • Open source (UTMStack, Wazuh, OpenSearch, OSSEC, Suricata, Snort, Zeek): you may run, change and redistribute the software under the license’s terms, including for customers.
  • Free but source-available (Security Onion, Graylog Open, Elastic’s downloads): no invoice and readable source, but terms that restrict building a commercial service on top. For an internal team this changes little. For a managed service provider it decides whether the business plan is legal.
  • Free tiers of commercial SIEMs appear in the same search results but belong to neither group. They come with a daily ingestion cap, a trial clock or features that disappear at renewal. They are fine for evaluation and a poor base for a compliance obligation that runs for years.

At a glance: which tool fits?

ToolBest forLicenseOffer it as a managed service?Built-in responseCompliance reportsSetup effort
UTMStackAll-in-one SIEM with response and audit evidenceAGPL-3.0Yes (multi-tenancy in Enterprise)YesYes (2 frameworks free, 9 in Enterprise)Low
WazuhEndpoint-centric monitoringGPLv2YesActive response scriptsMappingsMedium
Security OnionNetwork threat huntingElastic License 2.0NoNoNoHigh
OpenSearchBuild your own, fully openApache 2.0YesNoNoHigh
Elastic StackSearch-first custom stackElastic License 2.0 (AGPL source option)Not from the standard downloadPaid tiersPaid tiersHigh
Graylog OpenLog management firstSSPLRestrictedNoPaid tiersMedium
Suricata / Snort / ZeekNetwork sensors for any SIEMGPLv2 / GPLv2 / BSDYesNoNoMedium

The hidden cost of “free”

None of these tools charges a license fee, but every one of them has a cost of ownership:

  • Hardware. Months of logs need servers and storage, and search speed depends on fast disks.
  • People. Someone has to patch, tune rules, add log sources and investigate what the tool raises.
  • Compliance. Unless the tool produces the reports, you build them, and redo them before every audit.
  • The second year. The real risk is not a failed rollout. It is a successful one that nobody has time to maintain a year later, after the engineer who built it moves on.

This is why the amount of assembly matters as much as the license. A tool that arrives with correlation, response and reports built in needs fewer engineering hours to keep running than one you assemble from parts. If you outgrow running it yourself, UTMStack is also available as a hosted cloud service or with an Enterprise license priced by device rather than by log volume, so turning on more log sources does not raise the bill.

How to choose

  1. Decide whether you will offer it to customers. If you are a managed service provider, rule out Security Onion, Graylog Open and Elastic’s standard downloads first.
  2. Name your first problem. Endpoint visibility points to Wazuh, network hunting to Security Onion, log search to Graylog, and the full detect-respond-report loop to UTMStack.
  3. Count the people you have. OpenSearch and Elastic reward teams with engineers to spare; UTMStack and Wazuh need the least assembly.
  4. Check the audit requirement. If a framework such as PCI DSS, HIPAA or CMMC drives the project, favor a tool that produces the evidence rather than one where you build it.
  5. Test with your own logs. Every tool here is free to try, so run your two finalists side by side for a week before you commit.

Frequently asked questions

What is the best open source SIEM in 2026? It depends on the job. For a complete SIEM with correlation, automated response and compliance reports in one install, UTMStack. For endpoint-centric monitoring, Wazuh. For network threat hunting by an internal team, Security Onion. For a fully open search platform you build on, OpenSearch.

Are open source SIEM tools really free? The software is. Running it is not: you pay for servers, storage and the people who maintain and tune it, and for building compliance reports if the tool does not produce them. Budget for the second year, not just the first.

Which of these have a genuinely open source license? UTMStack (AGPL-3.0), Wazuh (GPLv2), OpenSearch (Apache 2.0), OSSEC, Suricata and Snort (GPLv2) and Zeek (BSD). Elasticsearch and Kibana source code can be used under the AGPL since 2024, but Elastic’s own downloads remain under the Elastic License. Security Onion (Elastic License 2.0) and Graylog Open (SSPL) are free to run but not open source.

Can a managed service provider build a service on these? On UTMStack, Wazuh, OpenSearch and the network sensors, yes. Security Onion’s license forbids offering it as a hosted or managed service, and Graylog’s SSPL and Elastic’s license restrict it. With UTMStack’s AGPL-3.0, running it for customers is fine; if you modify the code, you share those changes with the people using it. Native multi-tenancy is part of UTMStack Enterprise and its partner program.

Which open source SIEM is best for a home lab? Wazuh or OSSEC run on the least hardware. UTMStack and Security Onion want a dedicated machine with more memory, but install everything in one go, so they make a good lab for learning a complete security operations workflow.

Which open source SIEM includes compliance reports? UTMStack produces compliance reports from your own logs: CMMC 2.0 and GDPR in the free edition, and nine frameworks, including PCI DSS, HIPAA, ISO 27001 and SOC 2, in Enterprise. Wazuh maps its alerts to several frameworks. With the others you build the reports yourself.

Try UTMStack

The quickest way to judge it is to click through it. The live demo runs the full console in your browser on sample data, with no sign-up. When you are ready, install the free edition on your own server, or talk to an engineer about a cloud instance or an Enterprise license. For a longer list, including tools for detection engineering labs, see our top 10 open source SIEM tools.

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