Mask Your Web Server for Enhanced Security

Masking or anonymizing a Web server involves removing identifying details that intruders could use to detect your OS and Web server vendor and version. This information, while providing little or no utility to legitimate users, is often the starting place for crackers, blackhat hackers and "script kiddies". This article explores some ways you can minimize the risk of such detection. Most of the following examples focus on Microsoft’s Internet Information Server (IIS), since it has been most widely lambasted for its vulnerabilities, but some Apache detection countermeasures are also covered. While IIS users probably have the most vested interest here, server anonymization is relevant to anyone responsible for administering a Web server.

Crackers Start Here. Shouldn’t You?
Let’s look at it from the attacker's point of view: Security vulnerabilities tend to be dependent on software vendor and version. Blind probing might lead to further requests being denied or a system temporarily taken off line. Knowing Web server details greatly increases the efficiency of any attack. If an attacker can target exploits, the chances of successful cracking prior to detection increase significantly. Script kiddies can leverage canned, newly-discovered exploits to do more damage faster by targeting hosts with recognizable signatures. A self-identifying system invites trouble.

Port80 Software has developed an IIS server module called ServerMask to combat the majority of issues explored here for the Windows Web Server.

The Server Header Tells All
Most Web servers politely identify themselves and the OS to anyone who asks. Using a network query tool like Sam Spade or this Header Check, you can discern the HTTP Server header. Just request a Web site's home page and examine the resulting HTTP headers or "banners" sent back by the server. Among them, you will likely find something like this:

Server: Microsoft-IIS/5.0

There is not much mystery here. Apache's default settings make it no less identifiable:

Server: Apache/2.0.41-dev (UNIX)

You can remove or obscure this HTTP Server header in a variety of ways, depending on your platform. Apache 2.x users who have the mod_headers module loaded can use a simple directive in their httpd.conf file, as follows:

Header set Server "New Server Name Goes Here"

Unfortunately, mod_headers cannot alter the Server header in prior versions of Apache, so 1.3.x users will have to resort to editing the defines in httpd.h and recompiling Apache to get the same result. IIS users can install IISLockDown and use the configuration option in URLScan's INI file for removing or replacing the header. Be careful with URLScan if you are using Cold Fusion application server -- the way the current version replaces the Server header wreaks havoc with CFM pages. In fact, removing the header is the way to go when using URLScan, since if you try replacing the header it moves to the bottom of the header order -- which pretty much gives away that you are running URLScan on IIS.