Improper error handling

Every piece of information an attacker receives about a targeted system or application is a valuable weapon. It is the job of application designers and programmers to keep these weapons from the hands of the enemy. Unfortunately, many Web applications cannot handle errors and, as a result, improper error handling occurs.

Because they contain valuable debugging information, error messages are intended for those who figure out and fix problems. Trouble arises when end users see these messages and use the information to solicit an attack. Here are four ways error messages can create security problems:

So, what is a programmer to do? Do not rely on the operating system, server, database or other underlying packages to provide error handling. All errors should be handled by code the programmer writes. A well-designed application should include handlers for anticipated error conditions and a failsafe error handler for unanticipated conditions.

Error handlers should capture relevant, detailed information in a secure log for future analysis and present users with a generic error message that does not contain sensitive information. When designing the log file, keep security in mind. It should capture in

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Having an error log is not enough, however. Someone will need to look at the log and analyze its contents. Many application-level reconnaissance and attacks can be detected by looking for patterns in log files. If a log file shows many error conditions falling through to the default exception handler, it may be time to update the error-handling code to deal with other conditions.

These best practices require the application designer and programmer to think like an attacker. When designing an error message, stop and think about whether it is presenting information the user shouldn't see or that an attacker could use.

Owners of Internet-based services and businesses are at war with those who seek to attack their applications -- don't give them additional information and weapons by saving a little time on error handling!

About the Author
Al Berg, CISSP, CISM is Information Security Director of New York City based Liquidnet (www.liquidnet.com). Liquidnet is the leading electronic venue for institutional block equities trading and the 4th fastest growing privately held financial services company in the US.

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