Add warning regarding server host configuration (dotnet#4634)

Author: scottaddie

xml/System.Net/HttpListener.xml

@@ -38,6 +38,9 @@
  When a port is specified, the host element can be replaced with "\*" to indicate that the <xref:System.Net.HttpListener> accepts requests sent to the port if the requested URI does not match any other prefix. For example, to receive all requests sent to port 8080 when the requested URI is not handled by any <xref:System.Net.HttpListener>, the prefix is *http://\*:8080/*. Similarly, to specify that the <xref:System.Net.HttpListener> accepts all requests sent to a port, replace the host element with the "+" character. For example, *https://+:8080*. The "\*" and "+" characters can be present in prefixes that include paths.
   
  Starting with .NET Core 2.0 or .NET Framework 4.6 on Windows 10, wildcard subdomains are supported in URI prefixes that are managed by an <xref:System.Net.HttpListener> object. To specify a wildcard subdomain, use the "\*" character as part of the hostname in a URI prefix. For example, *http://\*.foo.com/*. Pass this as the argument to the <xref:System.Net.HttpListenerPrefixCollection.Add%2A> method. This works as of .NET Core 2.0 or .NET Framework 4.6 on Windows 10; in earlier versions, this generates an <xref:System.Net.HttpListenerException>.
+
+ > [!WARNING]
+ > Top-level wildcard bindings (*http://\*:8080/* and *http://+:8080*) should **not** be used. Top-level wildcard bindings can open up your app to security vulnerabilities. This applies to both strong and weak wildcards. Use explicit host names rather than wildcards. Subdomain wildcard binding (for example, `*.mysub.com`) doesn't have this security risk if you control the entire parent domain (as opposed to `*.com`, which is vulnerable). See [rfc7230 section-5.4](https://tools.ietf.org/html/rfc7230#section-5.4) for more information.
   
  To begin listening for requests from clients, add the URI prefixes to the collection and call the <xref:System.Net.HttpListener.Start%2A> method. <xref:System.Net.HttpListener> offers both synchronous and asynchronous models for processing client requests. Requests and their associated responses are accessed using the <xref:System.Net.HttpListenerContext> object returned by the <xref:System.Net.HttpListener.GetContext%2A> method or its asynchronous counterparts, the <xref:System.Net.HttpListener.BeginGetContext%2A> and <xref:System.Net.HttpListener.EndGetContext%2A> methods.