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Revision as of 14:49, 12 January 2009
OWASP Code Review Guide Table of ContentsError Handling
Error Handling is important in a number of ways. It may affect the state of the application, or leak system information to a user. The initial failure to cause the error may cause the application be traverse into an insecure state. 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 when developing code. This approach makes it more difficult for attackers to identify signatures of potentially successful attacks. There are methods which can circumvent systems with leading practice error handling semantics which should be kept in mind; Attacks such as blind SQL injection using booleanization or response time characteristics can be used to address such generic responses.
The other key area relating to error handling is the premise of "fail securely". 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).
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 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 strategy is important for three reasons:
- Good error handling does not give an attacker any information which is a means to an end, attacking the application
- 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.
- 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 errors 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.
Error Handling Should Be Centralized if Possible
When reviewing code it is recommended to 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.
This prevents the attacker identifying internal responses to error states. This also makes it more difficult for automated tools to identify successful attacks.
Declarative Exception Handling
<exception key=”bank.error.nowonga”
path=”/NoWonga.jsp”
type=”mybank.account.NoCashException”/>
This could be found in the struts-config.xml file, a key file when reviewing the wired-up struts environment
Java Servlets and JSP
Specification can be done in web.xml in order to handle unhandled exceptions. When Unhandled exceptions occur, but not caught in code, the user is forwarded to a generic error page:
<error-page>
<exception-type>UnhandledException</exception-type>
<location>GenericError.jsp</location>
</error-page>
Also in the case of HTTP 404 or HTTP 500 errors during the review you may find:
<error-page> <error-code>500</error-code> <location>GenericError.jsp</location> </error-page>
Failing Securely
Types of errors:
- The result of business logic conditions not being met.
- The result of the environment wherein the business logic resides fails.
- The result of upstream or downstream systems upon which the application depends fail.
- Technical hardware / physical failure
A failure is never expected, but they do occur such like much in life. 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 down stream server not being contactable.
Such areas of failure should be examined during the course of the code review. It should be examined if all resources should be released in the case of a failure and during the thread of execution if there is any potential for resource leakage, resources being 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;
e.g: "A logged in user looking to access an account which is not registered to that user and such data could not be inputted in the normal fashion."
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).
Information burial
Swallowing exceptions into an empty catch() block is not advised as an audit trail of the cause of the exception would be incomplete.
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 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.
How to locate the potentially vulnerable code
JAVA
IIn 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 important to know is that the output of these methods is printed in System console, the same as System.out.println(e) where e is an Exception. Be sure to not redirect the outputStream to PrintWriter object of JSP, by convention called "out". Ex. printStackTrace(out);
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.
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:
- On the Page in the Page_Error sub.
- The global.asax Application_Error sub
- The web.config file
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 make 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
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.
<script language="C#" runat="server"> Sub Page_Error(Source As Object, E As EventArgs) Dim message As String = "<font face=verdana color=red> " & Request.Url.ToString() & "<font color="red'>" & Server.GetLastError().ToString()& "</font>" 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 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 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. As long as 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.
<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. 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. 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>
Leading Practice 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.
In classic ASP there are 2 ways to do error handling, the first is using the err object with an On Error Resume Next
Public Function IsInteger (ByVal Number) Dim Res, tNumber Number = Trim(Number) tNumber=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, to use this method please go to the following URL: http://support.microsoft.com/kb/299981
Dim ErrObj set ErrObj = Server.GetLastError() 'Now use ErrObj as the regular err object
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 the 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:
- 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 >
Classic ASP
For classic ASP pages you need to do some IIS configuration, follow the same link for more information http://support.microsoft.com/kb/299981
