Você pode usar mod_rewrite para realizar essa tarefa:
RewriteEngine On
RewriteCond %{HTTP:X-Forwarded-Host} cdn.example.com
RewriteRule .* - [F]
Como você pode ver, se você testá-lo com o curl, o acesso será negado:
[root@RPX conf]# curl -i -H "X-Forwarded-Host: cdn.example.com" http://localhost:80/algo/
HTTP/1.1 403 Forbidden
Date: Tue, 02 Jun 2015 16:49:37 GMT
Server: Apache
Content-Length: 207
Content-Type: text/html; charset=iso-8859-1
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>403 Forbidden</title>
</head><body>
<h1>Forbidden</h1>
<p>You don't have permission to access /algo/
on this server.</p>
</body></html>