Security Onion is a free and open platform. The vast majority of software included in Security Onion is licensed under OSI-approved open source licenses. However, starting in Elastic 7.11, the Elastic Stack is licensed under the Elastic License. When you install or upgrade to Security Onion 2.3.40 or higher, you will be prompted to accept the Elastic License:
You can read more about the Elastic License at https://www.elastic.co/licensing/elastic-license.
For more information about the Elastic license change, please see https://securityonion.net/elastic-license.