Add some security checks when handling a websocket connection.

Check the Host and Origin headers for the incoming connection to
verify the connection is allowed.

The language is intended to allow the specific behaviour to be largely
implementation defined, whilst recommending a default
behaviour that prevents CSRF-type attacks (reject host headers that
aren't an IP address or the server hostname, reject any requests with
an origin header).

Hopefully adding this text will ensure that implementations consider
the security issues accepting a connection, even though it's not
possible to give precise requirements that apply to all
implementations.

index.html

@@ -428,8 +428,52 @@ <h3>Processing model</h3>
   received data, according to the requirements of [[RFC7230]]. If it
   is not possible to construct a complete <a>HTTP request</a>,
   the <a>remote end</a> must either close the <a>connection</a>,
-  return an HTTP response with status code 500, or return
-  an <a>error</a> with <a>error code</a> <a>unknown error</a>.
+  return an HTTP response with status code 500, or <a>send an
+  error</a> with <a>error code</a> <a>unknown error</a>, and then
+  jump to step 1.
+
+  <li><p><p>If <var>request</var> has a <a>Host header</a>,
+  let <var>host</var> be the value of that header. Otherwise
+  let <var>host</var> be null.</p></li>
+
+  <li><p><p>If <var>request</var> has an <a>Origin header</a>,
+  let <var>origin</var> be the value of that header. Otherwise
+  let <var>origin</var> be null.</p></li>
+
+  <li><p>If any of the following conditions hold:</li>
+   <ul>
+    <li><p><var>host</var> doesn't match the <code>Host</code>
+      grammar [[RFC7230]]</p></li>
+    <li><p>The result of <a>host parsing</a> the <code>uri-host</code>
+        part of <var>host</var> is not an <a>IP address</a>,
+        a <a>domain</a> identical to the
+        <a>host</a> of the HTTP server or to another <a>host</a> the
+        implementation has been configured to allow.</p></li>
+    <li><p>The <code>port</code> part of <var>host</var> is present
+        but doesn't match the port of the HTTP server.</p></li>
+    <li><p>The <code>port</code> part of <var>host</var> is not
+        present, and the port of the HTTP server doesn't match the
+        default port for the request's scheme.</p></li>
+    <li><p> implementation wants to reject
+        connections with <var>host</var> as the <a>Host header</a>.</li></p>
+   </ul>
+   <p>Then <a>send an error</a> with <a>error code</a> <a>unknown
+  error</a>, and jump to step 1.</p>
+
+  <p class=note>Rejecting connections with unexpected values in the
+  <a>Host header</a> prevents DNS rebinding attacks. Implementations can opt
+  to provide more stringent controls where appropriate, for example
+  only accepting connections when the <var>host</var> value
+  corresponds to a loopback interface [[RFC5735]].</p></li>
+
+  <li><p>If <var>origin</var> is not null, and is not identical to an
+  <a>Origin header</a> value that the implementation has been
+  configured to allow, then stop running these steps and act as if the
+  requested service is not available.</p>
+
+  <p class=note>Rejecting connections with unexpected values in
+  the <a>Origin header</a> is necessary to prevent untrusted websites
+  from establishing a WebDriver session.</p></li>
 
  <li><p>Let <var>request match</var> be the result of the algorithm
   to <a>match a request</a> with <var>request</var>’s
@@ -10958,6 +11002,16 @@ <h2>Index</h2>
   it is supposed that the implementation supports the relevant subsets of
   [[RFC7230]], [[RFC7231]], [[RFC7232]], [[RFC7234]], and [[RFC7235]].
 
+ <dd><p>The following terms are defined in the Web Origin Concept specification: [[RFC6454]]
+  <ul>
+   <!-- Origin header --> <li><dfn><a href="https://datatracker.ietf.org/doc/html/rfc6454#section-7">Origin header</a></dfn>
+  </ul>
+
+ <dd><p>The following terms are defined in the Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing specification: [[RFC7230]]
+  <ul>
+   <!-- Host header --> <li><dfn><a href="https://datatracker.ietf.org/doc/html/rfc7230#section-5.4">Host header</a></dfn>
+  </ul>
+
  <dd><p>The following terms are defined in the Cookie specification: [[RFC6265]]
   <ul>
    <!-- Compute cookie-string --> <li><dfn><a href=https://tools.ietf.org/html/rfc6265#section-5.4>Compute <code>cookie-string</code></a></dfn>