AWS WAF now supports configuring the HTTP status code and the response body returned to the user when a request is blocked. Until today, AWS WAF could only return HTTP status code 403 (forbidden) when the user request was blocked by WAF. With Custom Response, you can now configure AWS WAF to send out a different HTTP status code, such as 3xx (redirects), 4xx (client errors), or 5xx (server errors). These codes can be used to redirect users to different parts of your application or provide users a specific response code based on the reason they were blocked by WAF. In addition, you can use Custom Response to include a response body to present a customized error message back to the user.
Source:: Amazon AWS