Error Handling

OWASP Code Review Guide Table of Contents

Error, Exception handling & Logging.

Contact author: Eoin Keary

An important aspect of secure application development is to prevent information leakage. Error messages give an attacker great insight into the inner workings of an application.

The purpose of reviewing the Error Handling code is to assure the application fails safely under all possible error conditions, expected and unexpected. No sensitive information is presented to the user when an error occurs.

For example SQL injection is much tougher to successfully pull off without some healthy error messages. It lessens the attack footprint and our attacker would have to resort to use “blind SQL injection” which is more difficult and time consuming.

A well-planned error/exception handling strategy is important for three reasons:

1. Good error handling does not give an attacker any information which is a means to an end, attacking the application
2. A proper centralised error strategy is easier to maintain and reduces the chance of any uncaught errors “Bubbling up” to the front end of an application.
3. Information leakage can lead to social engineering exploits.

Some development languages provide checked exceptions which mean that the compiler shall complain if an exception for a particular API call is not caught Java and C# are good examples of this. Languages like C++ and C do not provide this safety net. Languages with checked exception handling still are prone to information leakage as not all types of error are checked for.

When an exception or error is thrown we also need to log this occurrence. Sometimes this is due to bad development, but it can be the result of an attack or some other service your application relies on failing.

All code paths that can cause an exception to be thrown should check for success in order for the exception not to be thrown.

To avoid a NullPointerException we should check is the object being accessed is not null.

Generic error messages

We should use a localized description string in every exception, a friendly error reason such as “System Error – Please try again later”. When the user sees an error message, it will be derived from this description string of the exception that was thrown, and never from the exception class which may contain a stack trace, line number where the error occurred, class name or method name.

Do not expose sensitive information in exception messages. Information such as paths on the local file system is considered privileged information; any system internal information should be hidden from the user. As mentioned before an attacker could use this information to gather private user information from the application or components that make up the app.

Don’t put people’s names or any internal contact information in error messages. Don’t put any “human” information, which would lead to a level of familiarity and a social engineering exploit.

How to locate the potentially vulnerable code

JAVA

In java we have the concept of an error object, the Exception object. This lives in the java package java.lang and is derived from the Throwable object Exceptions are thrown when an abnormal occurrence has occurred. Another object derived from Throwable is the Error object, which is thrown when something more serious occurs.

Information leakage can occur when developers use some exception methods, which ‘bubble’ to the user UI due to a poor error handling strategy. The methods are as follows: printStackTrace() getStackTrace()

Also another object to look at is the java.lang.system package:

setErr() and the System.err field.

.NET

In .NET a System.Exception object exists. Commonly used child objects such as ApplicationException and SystemException are used. It is not recommended that you throw or catch a SystemException this is thrown by runtime.

When an error occurs, either the system or the currently executing application reports it by throwing an exception containing information about the error, similar to java. Once thrown, an exception is handled by the application or by the default exception handler. This Exception object contains similar methods to the java implementation such as:

StackTrace Source Message HelpLink

In .NET we need to look at the error handling strategy from the point of view of global error handling and the handling of unexpected errors. This can be done in many ways and this article is not an exhaustive list. Firstly an Error Event is thrown when an unhandled exception is thrown. This is part of the TemplateControl class.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfSystemWebUITemplateControlClassErrorTopic.asp

Error handling can be done in three ways in .NET

• In the web.config file's customErrors section.
• In the global.asax file's Application_Error sub.
• On the aspx or associated codebehind page in the Page_Error sub

The order of error handling events in .NET is as follows:

1. On the Page in the Page_Error sub.
2. The global.asax Application_Error sub
3. The web.config file

It is recommended to look in these areas to understand the error strategy of the application.

Vulnerable Patterns for Error Handling

Page_Error

Page_Error is page level handling which is run on the server side. Below is an example but the error information is a little too informative and hence bad practice.

FIXME: code formatting

<script language="C#" runat="server"> Sub Page_Error(Source As Object, E As EventArgs) Dim message As String = "<font face=verdana color=red><h1>" & Request.Url.ToString()& "</h1>" & "<pre><font color='red'>" & Server.GetLastError().ToString()& "</pre></font>" Response.Write(message) // display message End Sub </script>

The red text in the example above has a number of issues: Firstly it redisplays the HTTP request to the user in the form of Request.Url.ToString() Assuming there has been no data validation prior to this point we are vulnerable to cross site scripting attacks!! Secondly the error message and stack trace is displayed to the user using Server.GetLastError().ToString() which divulges internal information regarding the application.

After the Page_Error is called, the Application_Error sub is called:

Global.asax

When an error occurs, the Application_Error sub is called. In this method we can log the error and redirect to another page.

<%@ Import Namespace="System.Diagnostics" %>
  <script language="C#" runat="server">
    void Application_Error(Object sender, EventArgs e) {
         String Message = "\n\nURL: http://localhost/" + Request.Path
                          + "\n\nMESSAGE:\n " + Server.GetLastError().Message
                          + "\n\nSTACK TRACE:\n" + Server.GetLastError().StackTrace;
         // Insert into Event Log
         EventLog Log = new EventLog();
         Log.Source = LogName;
         Log.WriteEntry(Message, EventLogEntryType.Error);
       Server.Redirect(Error.htm) // this shall also clear the error
    }
</script>

Above is an example of code in Global.asax and the Application_Error method. The error is logged and then the user is redirected. Unvalidated parameters are being logged here in the form of Request.Path. Care must be taken not to log or redisplay unvalidated input from any external source.

Web.config

Web.config has a custom errors tag which can be used to handle errors. This is called last and if Page_error or Application_error called and has functionality that functionality shall be executed first. As long as the previous two handling mechanisms do not redirect or clear (Response.Redirect or a Server.ClearError) this shall be called. And you shall be forwarded to the page defined in web.config

<customErrors defaultRedirect="error.html" mode="On|Off|RemoteOnly">
   <error statusCode="statuscode" redirect="url"/>
</customErrors>

The “On" directive means that custom errors are enabled. If no defaultRedirect is specified, users see a generic error. "Off" directive means that custom errors are disabled. This allows display of detailed errors. "RemoteOnly" specifies that custom errors are shown only to remote clients, and ASP.NET errors are shown to the local host. This is the default.

<customErrors mode="On" defaultRedirect="error.html">
    <error statusCode="500" redirect="err500.aspx"/>
    <error statusCode="404" redirect="notHere.aspx"/>
    <error statusCode="403" redirect="notAuthz.aspx"/>
</customErrors>

Best Practices for Error Handling

Try & Catch (Java/ .NET)

Code that might throw exceptions should be in a try block and code that handles exceptions in a catch block. The catch block is a series of statements beginning with the keyword catch, followed by an exception type and an action to be taken. These are very similar in Java and .NET

Example:

Java Try-Catch:

public class DoStuff {
    public static void Main() {
        try {
            StreamReader sr = File.OpenText("stuff.txt");
            Console.WriteLine("Reading line {0}", sr.ReadLine());    
        }
        catch(Exception e) {
            Console.WriteLine("An error occurred. Please leave to room”);
	 logerror(“Error: “, e);
        }
    }
}

.NET try – catch

public void run() {
            while (!stop) {
                try {

                    // Perform work here

                } catch (Throwable t) {
                    // Log the exception and continue
		WriteToUser(“An Error has occurred, put the kettle on”);
                    logger.log(Level.SEVERE, "Unexception exception", t);
                }
            }
        }

In general, it is best practice to catch a specific type of exception rather than use the basic catch(Exception) or catch(Throwable) statement in the case of Java.

Releasing resources and good housekeeping

If the language in question has a finally method use it. The finally method is guaranteed to always be called. The finally method can be used to release resources referenced by the method that threw the exception. This is very important. An example would be if a method gained a database connection from a pool of connections and an exception occurred without finally the connection object shall not be returned to the pool for some time (until the timeout). This can lead to pool exhaustion. finally() is called even if no exception is thrown.

try {
       System.out.println("Entering try statement");
       out = new PrintWriter(new FileWriter("OutFile.txt"));
     //Do Stuff….

   } catch (Exception e) {
       System.err.println("Error occurred!”);

   } catch (IOException e) {
       System.err.println("Input exception ");

   } finally {

       if (out != null) { 
           out.close(); // RELEASE RESOURCES
       } 
   }

A Java example showing finally() being used to release system resources.

Centralised exception handling (Struts Example)

Building an infrastructure for consistent error reporting proves more difficult than error handling. Struts provides the ActionMessages & ActionErrors classes for maintaining a stack of error messages to be reported, which can be used with JSP tags like <html: error> to display these error messages to the user.

To report a different severity of a message in a different manner (like error, warning, or information) the following tasks are required:

• Register, instantiate the errors under the appropriate severity
• Identify these messages and show them in a constant manner.

Struts ActionErrors class makes error handling quite easy:

ActionErrors errors = new ActionErrors()
errors.add("fatal", new ActionError("....")); 
errors.add("error", new ActionError("....")); 
errors.add("warning", new ActionError("...."));
errors.add("information", new ActionError("....")); 
saveErrors(request,errors); // Important to do this

Now we have added the errors we display them by using tags in the HTML page.

<logic:messagePresent property="error"> 
<html:messages property="error" id="errMsg" >
    <bean:write name="errMsg"/>
</html:messages>
</logic:messagePresent >