The new CORS Violation Record event type in the EventLogFile object captures information about Cross-Origin Resource Sharing (CORS) violations for Lightning apps. Cross-origin requests to Lightning apps are blocked unless the request comes from a URL listed in your CORS allowlist. Use this event type to monitor CORS violation records and help you decide if your CORS allowlist requires an update before the Enforce CORS Allowlist for Lightning Apps release update is enforced. Currently, the CORS allowlist in Setup isn’t enforced for Lightning apps.
Where: This change applies to Enterprise, Performance, Unlimited, and Developer editions. This event is available in the API but not in the Event Monitoring Analytics app. It’s available for free for two releases because it’s intended to help you prepare for the enforcement of the Enforce CORS Allowlist for Lightning Apps release update.
How: CORS enables web browsers to request resources from origins other than their own. For example, using CORS, JavaScript code at https://www.example.com can request a resource from https://www.salesforce.com.
The Event Log File Browser application is the easiest tool to download event monitoring data. See these instructions.
Get Information About CORS Violation Records in the EventLogFile (salesforce.com)