Prevent XML DoS attack in ASP.NET webmethod

Question

I have a following web method (ASP.NET, c# 4.0):

[WebMethod]
public XmlDocument NewQuote(string username, string password, XmlDocument xml)
{
   // process request
}

I was wondering how to prevent XML DoS attack (billion laughs, external entity, etc.), as XmlDocument is accepted as a parameter? Where and how do I prohibit DTD processing? It is obviously too late in the body of the web method.

Thanks!

Answer 1

Put a logic that the user can only upload x amount of files or call that service x amount of times.