The default HTTP header created by ASP.NET includes a string called X-AspNet-Version. An example....
Ideally you want to give away as little information as possible about your web server to potential hackers. You can easily remove this string by making a change to the web.config file.
Setting the enableVersionHeader attribute to true in the httpRuntime element will remove the string from all the http headers.
<httpRuntime enableVersionHeader="false" />
.NET Slave blog post.