The request requires user authentication. The response MUST include a WWW-Authenticate header field (section 14.47) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section 14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP access authentication is explained in rfc2617: http://www.ietf.org/rfc/rfc2617.txt (description is taken from http://www.submissionchamber.com/help-guides/error-codes.php#sec10.4.2)

public class UnauthorizedException : HttpException

Public Class UnauthorizedException _
	Inherits HttpException

public ref class UnauthorizedException : public HttpException

System..::.Object
  System..::.Exception
    HttpServer.Exceptions..::.HttpException
      HttpServer.Exceptions..::.UnauthorizedException