Error '503 service unavailable' when URL contain special characters

Why 503 and Not 404?

The request validation process checks the URL name before performing any resource lookup. When the URL contains a disallowed special character, the name validation fails at the transport layer and the request is rejected with a 503 status code. Because no resource lookup occurs, a 404 response is never generated.

Example

• Accessing https://subdomain.example.com/ works correctly and returns the expected page.
• Accessing https://subdomain.example.com/< returns a 503 Service Unavailable error because the < character fails the URL name validation check.

How to Diagnose

To confirm the 503 status code, open browser developer tools (press F12), navigate to the Network tab, and reload the failing URL. Inspect the HTTP status code of the failed request — it shows 503 rather than 404, confirming the URL validation failure.

Resolution Steps

Remove all special characters from the site URL path. The following characters are not permitted in Salesforce Site URLs: < > ( ) { } [ ] !
Ensure the site URL path uses only alphanumeric characters, hyphens, and forward slashes.