Secure programmer: Validating input (IBM developerWorks)
One of the biggest mistakes developers of secure programs make is to try to check for 'illegal' data values. It's a mistake because attackers are quite clever; they can often think of yet another dangerous data value. Instead, determine what is legal, check if the data matches that definition, and reject anything that doesn't match that definition. For security it's best to be extremely conservative to start with, and allow just the data that you know is legal. After all, if you're too restrictive, users will quickly report that the program won't allow legitimate data to be entered. On the other hand, if you're too permissive, you may not find that out until after your program has been subverted."