The trace feature of ASP.NET is one of the most useful tools that you can use to ensure application security by debugging and profiling your Web-based applications. Unfortunately, it is also one of the most useful tools that a hacker can use to attack your Web-based applications if it is left enabled in a production environment.

Vulnerable configuration:

<configuration> 
  <system.web> 
    <trace enabled="true" localOnly="false"> 

Secure configuration:

<configuration> 
  <system.web> 
    <trace enabled="false" localOnly="true"> 

When the <trace> element is enabled for remote users of Web-based applications (localOnly="false"), any user can view an incredibly detailed list of recent requests to the application simply by browsing to the page "trace.axd." If a detailed exception message is like a gold mine to a hacker looking to circumvent application security, a trace log is like Fort Knox! A trace log presents a wealth of information: the .NET and ASP.NET versions that the server is running; a complete trace of all the page methods that the request caused, including their times of execution; the session state and application state keys; the request and response cookies; the complete set of request headers, form variables, and QueryString variables; and finally the complete set of server variables.

A hacker looking for a way around application security would obviously find the form variable histories useful because these might include email addresses that could be harvested and sold to spammers, IDs and passwords that could be used to impersonate the user, or credit card and bank account numbers. Even the most innocent-looking piece of data in the trace collection can be dangerous in the wrong hands. For example, the "APPL_PHYSICAL_PATH" server variable, which contains the physical path of Web-based applications on the server, could help an attacker perform directory traversal attacks against the system.

The best way to prevent a hacker from obtaining trace data from Web-based applications is to disable the trace viewer completely by setting the "enabled" attribute of the <trace> element to "false." If you have to have the trace viewer enabled, either to debug or to profile your application, then be sure to set the "localOnly" attribute of the <trace> element to "true." That allows users to access the trace viewer only from the Web server and disables viewing it from any remote machine, increasing your application security.