(Sorry about the shameless plug; I don't think I've advertised it here yet.)
jub0bs/cors is my latest CORS middleware library for Go. Here are some reasons for favouring it over the more popular rs/cors library:

• a simpler API
• better documentation
• extensive configuration validation (of origins, header names, etc. and more!)
• a useful debug mode (for ease of troubleshooting CORS errors)
• dynamic, concurrency-safe reconfigurability (no need for a server restart)
• stronger performance guarantees