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In the HTTP protocol used on the World Wide Web, 403 Forbidden is an HTTP status code returned by a web server when a user requests a web page or media that the server does not allow them to. In other words, the server can be reached, but the server declined to allow access to the page. This response is returned by the Apache web server when directory listings have been disabled. Microsoft IIS responds in the same way when directory listings are denied. (This response may also be returned by the server if the client issued a WebDAV PROPFIND request but did not also issue the required Depth header, or issued a Depth header of infinity.)[1]
403 Substatus Error Codes for IIS
403.1 - Execute access forbidden.
403.2 - Read access forbidden.
403.3 - Write access forbidden.
403.4 - SSL required.
403.5 - SSL 128 required.
403.6 - IP address rejected.
403.7 - Client certificate required.
403.8 - Site access denied.
403.9 - Too many users.
403.10 - Invalid configuration.
403.11 - Password change.
403.12 - Mapper denied access.
403.13 - Client certificate revoked.
403.14 - Directory listing denied.
403.15 - Client Access Licenses exceeded.
403.16 - Client certificate is untrusted or invalid.
403.17 - Client certificate has expired or is not yet valid.