Unable to connect to websocket gateway on a NestJS Application deployed via App Runner

There are several possible causes of a 403 error that may appear when attempting to connect to a WebSocket gateway in a NestJS application that has been set up on AWS App Runner. To troubleshoot and fix the issue, follow these steps:

Verify Network Configuration and Security Groups: Make sure that the WebSocket port that you have defined for incoming traffic is open in the security groups linked to your App Runner service. Make sure that no firewalls or network ACLs (access control lists) are preventing WebSocket connections.

Check WebSocket Endpoint: Make that the URL you are using to connect to the WebSocket endpoint corresponds to the settings of your deployed application.