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CodeReview - Error Handling

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Error Handling

Proper error handling is important in two ways: <are these the only 2 ways?>

  1. It may affect the state of the application
The initial failure to prevent the error may cause the application to traverse into an insecure state. This covers the key premise of failing securely <include reference to antipatterns>, errors induced should not leave the application in an insecure state. Resources should be locked down and released, sessions terminated (if required), and calculations or business logic should be halted (depending on the type of error, of course).
  1. It may leak system information to a user
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. Weak error handling also aids the attacker, as the errors returned may assist them in constructing correct attack vectors.

A generic error page for most errors is recommended, this approach makes it more difficult for attackers to identify signatures of potentially successful attacks such as blind SQL injection using booleanization <should include reference> or analysis of response time characteristics.

Reviewing Error Handling

The purpose of reviewing the Error Handling code is to assure that the application fails safely under all possible error conditions, expected and unexpected. No sensitive information is presented to the user when an error occurs. A company coding guidelines should include sections on Error Handling and how it should be controlled by an application suite, this will allow developers to code against this guidelines as well as review against them.

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

A well-planned error/exception handling guideline is important in a company 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 could lead to social engineering exploits <we should explain how this is done - i'm unsure how this is accomplished>.

Regardless of whether the development language provide checked exceptions or not, reviewers should remember:

  1. Not all errors are exceptions
Don't rely on exception handling to be your only way of handling errors, handle all case statement 'default' sections, ensure all 'if' statements have their 'else' clauses covered, ensure that all exits from a function (e.g. return statements, exceptions, etc) are covered. RAII concepts (e.g. auto pointers and the like) are an advantage here. In languages like Java and C#, remember that errors are different from exceptions (different hierarchy) and should be handled.
  1. Catching an exception is not automatically handling it
You've caught your exception, so how do you handle it? For many cases this should be obvious enough, based on your business logic, but for some (e.g. out of memory, array index out of bounds, etc) the handling many not be so simple.
  1. Don't catch more that you can handle
Catch all clauses (e.g. 'catch(Exception e)' in Java & C# or 'catch(...) in C++) should be avoided as you will not know what type of expcetion you are handling, and if you don't know the exception type, how do you accurately handle it? It could be that the downstream server is not responding, or a user may have exceeded their quota, or you may be out of memory, these issues should be handled in different ways and thus should be caught in exception clauses that are specific.

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. This has to be imagined in the production scenario, if your application handles 'failing securely' by returning an error response to the client, and since we don't want to leak informaiton that error will be generic, we need to have some way of identifying why the failure occured. If your customer reports 1000's of errors occured last night, you know that customer is going to want to know why. If you don't have proper logging and tracability coded into your application then you will not be able to establish if those errors were due to some attempted hack, or an error in your business logic when handling a particular type of error.

All code paths that can cause an exception to be thrown should check for success in order for the exception not to be thrown. This could be hard to impossible for a manual code review to cover, especially for large bodies of code. However if there is a debug version of the code, then modules/functions could throw relevant exceptions/errors and an automated tool can ensure the state and error responses from the module is as expected. This then means the code reviewer has the job of ensuring all relevant exceptions/errors are tested in the debug code.

Centralized Error Handling

When reviewing code it is recommended that you assess the commonality within the application from a error/exception handling perspective. Frameworks have error handling resources which can be exploited to assist in secure programming, and such resources within the framework should be reviewed to assess if the error handling is "wired-up" correctly. A generic error page should be used for all exceptions if possible as this prevents the attacker from identifying internal responses to error states. This also makes it more difficult for automated tools to identify successful attacks.

For JSP structs this could be controlled in the struts-config.xml file, a key file when reviewing the wired-up struts environment:

<exception   key=”bank.error.nowonga” 

Specification can be done for JSP in web.xml in order to handle unhandled exceptions. When unhandled exceptions occur, but are not caught in code, the user is forwarded to a generic error page:


Also in the case of HTTP 404 or HTTP 500 errors during the review you may find:


For IIS development the 'Application_Error()' handler will allow the application to catch all uncaught exceptions and handle them in a consistent way. Note this is important or else there is a chance your exception informaiton could be sent back to the client in the response.

For Apache development, returning failures from handlers or modules can prevent an further processing by the Apache engine and result in an error response from the server. Response headers, body, etc can be set by by the handler/module or can be configured using the "ErrorDocument" configuration.

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 like exception messages. Information such as paths on the local file system is considered privileged information; any internal system 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.

Failing Securely

There can be many different reasons why an application may fail, for example:

  • The result of business logic conditions not being met.
  • The result of the environment wherein the business logic resides fails <might need to explain exactly what this means, or give an example>.
  • The result of upstream or downstream systems upon which the application depends fail.
  • Technical hardware / physical failure.

Failures are like the Spanish Inquisition; popularly nobody expected the Spanish Inquisition (see Monty Python) but in real life the Spanish knew when an inquisition was going to occur and were prepared for it, similarly in an application, though you don't expect errors to occur your code should be prepared for them to happen. In the event of a failure, it is important not to leave the "doors" of the application open and the keys to other "rooms" within the application sitting on the table. In the course of a logical workflow, which is designed based upon requirements, errors may occur which can be programmatically handled, such as a connection pool not being available, or a downstream server returning a failure.

Such areas of failure should be examined during the course of the code review. It should be examined if resources should be released such as memory, connection pools, file handles etc.

The review of code should also include pinpointing areas where the user session should be terminated or invalidated. Sometimes errors may occur which do not make any logical sense from a business logic perspective or a technical standpoint, for example a logged in user looking to access an account which is not registered to that user. Such conditions reflect possible malicious activity. Here we should review if the code is in any way defensive and kills the user’s session object and forwards the user to the login page. (Keep in mind that the session object should be examined upon every HTTP request).

How to Locate the Potentially Vulnerable Code


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. The Error object can be caught in a catch clause, but cannot be handled, the best you can do is log some information about the Error and then re-throw it.

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:


Also important to know is that the output of these methods is printed in System console, the same as System.out.println(e) where there is an Exception. Be sure to not redirect the outputStream to PrintWriter object of JSP, by convention called "out", for example:


Note it is possible to change where system.err and system.out write to (like modifing fd 1 & 2 in bash or C/C++), using the java.lang.system package:

setErr() for the System.err field and setOut() for the System.out field.

This could be used on a process wide basis to ensure no output gets written to standard error or standard out (which can be reflected back to the client) but instead write to a configured log file.


In .NET a System.Exception object exists and has 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:


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, see

Error handling can be done in three ways in .NET, executed in the following order:

  • On the aspx or associated codebehind page in the Page_Error.
  • In the global.asax file's Application_Error (as mentioned before).
  • In the web.config file's customErrors section.

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

Classic ASP

Unlike Java and .NET, classic ASP pages do not have structured error handling in try-catch blocks. Instead they have a specific object called "err". This makes error handling in a classic ASP pages hard to do and prone to design errors on error handlers, causing race conditions and information leakage. Also, as ASP uses VBScript (a subtract of Visual Basic), sentences like "On Error GoTo label" are not available.



Vulnerable Patterns for Error Handling in IIS


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

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

The text in the example above has a number of issues:

  • Firstly, it displays 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.

<could maybe do with an example of this in Apache>


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

<surely we could do the same in Page_Error? With the previous advice on centralizing all errors, should we avoid a Page_Error example?>

<%@ 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

Above is an example of code in Global.asax and the Application_Error method. The error is logged and then the user is redirected. Non-validated parameters are being logged here in the form of Request.Path. Care must be taken not to log or display non-validated input from any external source. <link to XSS>


Web.config has custom error tags which can be used to handle errors. This is called last and if Page_error or Application_error is called and has functionality, that functionality shall be executed first. If the previous two handling mechanisms do not redirect or clear (Response.Redirect or a Server.ClearError), this will be called and you shall be forwarded to the page defined in web.config in the customErrors section, which is configured as follows:

<customErrors mode="<On|Off|RemoteOnly>" defaultRedirect="<default redirect page>">
 <error statusCode="<HTTP status code>" redirect="<specific redirect page for listed status code>"/>

The “On" directive means that custom errors are enabled whilst the "Off" directive means that custom errors are disabled. This allows the displaying of detailed errors. "RemoteOnly" specifies that custom errors are shown only to remote clients, and ASP.NET errors are shown to the local host. "RemoteOnly" is the default value for the defaultRedirect attribute. An example of the customErrors section completed for an application is as follows:

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

<clarify what defaultRedirect does>

Vulnerable Patterns for Error Handling in Apache


Leading Practice for Error Handling

Try & Catch (Java/.NET/C++)

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.

<the examples below don't handle the error, which is what we've been saying to do in this section, re-write the examples to show best practices>


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);

C++ Try–Catch


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.

<discuss the finally block, or the order in which these exceptions are listed?>

Classic ASP Error Handling

In classic ASP there are two ways to do error handling, the first is using the err object with a "On Error Resume Next" and "On Error GoTo 0".

Public Function IsInteger (ByVal Number)	 
  Dim Res, tNumber
  Number = Trim(Number)
  On Error Resume Next	                     'If an error occurs continue execution
  Number = CInt(Number) 	             'if Number is a alphanumeric string a Type Mismatch error will occur
  Res = (err.number = 0) 	             'If there are no errors then return true
  On Error GoTo 0			     'If an error occurs stop execution and display error
  re.Pattern = "^[\+\-]? *\d+$"	     'only one +/- and digits are allowed
  IsInteger = re.Test(tNumber) And Res
End Function

The second is using an error handler on an error page (

Dim ErrObj
set ErrObj = Server.GetLastError()
'Now use ErrObj as the regular err object

<changes done up to this point>

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.

Classic ASP

For Classic ASP pages it is recommended to enclose all cleaning in a function and call it into an error handling statement after an "On Error Resume Next".

Centralised exception handling (Struts Example)

Building an infrastructure for consistent error reporting proves more difficult than error handling. Struts provides the ActionMessages and 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:

  1. Register, instantiate the errors under the appropriate severity
  2. Identify these messages and show them in a consistent 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 that 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"/>
</logic:messagePresent >

Classic ASP

For classic ASP pages you need to do some IIS configuration, follow the same link for more information

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