Free - Top 10 Application Security Vulnerabilities in Web.config Files – Part One

This article is written by Bryan Sullivan and revised by Brian Cooper together with the DMX/DNzone team.

These days, the biggest threat to an organization's network security comes from its public Web site and the Web-based applications found there. Unlike internal-only network services such as databases—which can be sealed off from the outside via firewalls—a public Web site is generally accessible to anyone who wants to view it, making application security an issue. As networks have become more secure, vulnerabilities in Web applications have inevitably attracted the attention of hackers, both criminal and recreational, who have devised techniques to exploit these holes. In fact, attacks upon the Web application layer now exceed those conducted at the network level, and can have consequences which are just as damaging.

Some enlightened software architects and developers are becoming educated on these threats to application security and are designing their Web-based applications with security in mind. By "baking in" application security from the start of the development process, rather than trying to "brush it on" at the end, you are much more likely to create secure applications that will withstand hackers' attacks. However, even the most meticulous and security-aware C# or VB.NET code can still be vulnerable to attack if you neglect to secure the Web.config configuration files of your application. Incorrectly configured Web-based applications can be just as dangerous as those that have been incorrectly coded. To make matters worse, many configuration settings actually default to insecure values.

This article lists five of the "worst offenders" of misconfigurations of application security that are universally problematic for all ASP.NET Web-based applications. Part two of this article will list an additional five misconfigurations that are specifically applicable to ASP.NET sites that use Web Forms authentication. So without further ado, let's get started!

1. Custom Errors Disabled

When you disable custom errors as shown below, ASP.NET provides a detailed error message to clients by default.

Vulnerable configuration:

<configuration>
   <system.web>
      <customErrors mode="Off">

Secure configuration:

<configuration>
   <system.web>
      <customErrors mode="RemoteOnly">

In itself, knowing the source of an error may not seem like a risk to application security, but consider this: the more information a hacker can gather about a Web site, the more likely it is that he will be able to successfully attack it. An error message can be a gold mine of information to an attacker. A default ASP.NET error message lists the specific versions of ASP.NET and the .NET framework which are being used by the Web server, as well as the type of exception that was thrown. Just knowing which Web-based applications are used (in this case ASP.NET) compromises application security by telling the attacker that the server is running a relatively recent version of Microsoft Windows and that Microsoft Internet Information Server (IIS) 6.0 or later is being used as the Web server. The type of exception thrown may also help the attacker to profile Web-based applications; for example, if a SqlException is thrown, then the attacker knows that the application is using some version of Microsoft SQL Server.

You can build up application security to prevent such information leakage by modifying the mode attribute of the customErrors element to On or RemoteOnly. This setting instructs Web-based applications to display a nondescript, generic error message when an unhandled exception is generated. Another way to circumvent this application security issue is to redirect the user to a new page when errors occur by setting the defaultRedirect attribute of the customErrors element. This approach can provide even better application security because the default generic error page still gives away too much information about the system (namely, that it's using a Web.config file, which reveals that the server is running ASP.NET).

2. Leaving Tracing Enabled in Web-Based Applications

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.

Bryan Sullivan

Bryan SullivanBryan Sullivan is a development manager at SPI Dynamics (www.spidynamics.com), a Web application security products company. Bryan manages the DevInspect and QAInspect Web security products, which help programmers maintain application security throughout the development and testing process. He has a bachelor's degree in mathematics from Georgia Tech and 12 years of experience in the information technology industry. Bryan is currently coauthoring a book with noted security expert Billy Hoffman on Ajax security, which will be published in summer 2007 by Addison-Wesley.

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