Improper Error Handling

Contributor(s): Jeremy Ferragamo, Wichers, Jim Bird, kingthorin


Improper handling of errors can introduce a variety of security problems for a web site. The most common problem is when detailed internal error messages such as stack traces, database dumps, and error codes are displayed to the user (hacker). These messages reveal implementation details that should never be revealed. Such details can provide hackers important clues on potential flaws in the site and such messages are also disturbing to normal users.

Web applications frequently generate error conditions during normal operation. Out of memory, null pointer exceptions, system call failure, database unavailable, network timeout, and hundreds of other common conditions can cause errors to be generated. These errors must be handled according to a well thought out scheme that will provide a meaningful error message to the user, diagnostic information to the site maintainers, and no useful information to an attacker.

Even when error messages don’t provide a lot of detail, inconsistencies in such messages can still reveal important clues on how a site works, and what information is present under the covers. For example, when a user tries to access a file that does not exist, the error message typically indicates, “file not found”. When accessing a file that the user is not authorized for, it indicates, “access denied”. The user is not supposed to know the file even exists, but such inconsistencies will readily reveal the presence or absence of inaccessible files or the site’s directory structure.

One common security problem caused by improper error handling is the fail-open security check. All security mechanisms should deny access until specifically granted, not grant access until denied, which is a common reason why fail open errors occur. Other errors can cause the system to crash or consume significant resources, effectively denying or reducing service to legitimate users.

Good error handling mechanisms should be able to handle any feasible set of inputs, while enforcing proper security. Simple error messages should be produced and logged so that their cause, whether an error in the site or a hacking attempt, can be reviewed. Error handling should not focus solely on input provided by the user, but should also include any errors that can be generated by internal components such as system calls, database queries, or any other internal functions.

Environments Affected

All web servers, application servers, and web application environments are susceptible to error handling problems.

Examples and References

How to Determine If You Are Vulnerable

Typically, simple testing can determine how your site responds to various kinds of input errors. More thorough testing is usually required to cause internal errors to occur and see how the site behaves.

Another valuable approach is to have a detailed code review that searches the code for error handling logic. Error handling should be consistent across the entire site and each piece should be a part of a well-designed scheme. A code review will reveal how the system is intended to handle various types of errors. If you find that there is no organization to the error-handling scheme or that there appear to be several different schemes, there is quite likely a problem.

How to Protect Yourself

A specific policy for how to handle errors should be documented, including the types of errors to be handled and for each, what information is going to be reported back to the user, and what information is going to be logged. All developers need to understand the policy and ensure that their code follows it.

In the implementation, ensure that the site is built to gracefully handle all possible errors. When errors occur, the site should respond with a specifically designed result that is helpful to the user without revealing unnecessary internal details. Certain classes of errors should be logged to help detect implementation flaws in the site and/or hacking attempts. Very few sites have any intrusion detection capabilities in their web application, but it is certainly conceivable that a web application could track repeated failed attempts and generate alerts. Note that the vast majority of web application attacks are never detected because so few sites have the capability to detect them. Therefore, the prevalence of web application security attacks is likely to be seriously underestimated.