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Always Validate Input On The Server

In order to obtain an immediate response to incorrect form input, many developers employ client-side scripting. While this is a great tool and should certainly be employed where applicable, it's no substitute for good server-side validation.

Suppose a particlarly obstinate user doesn't like your validation code. What's to stop them from disabling javascript in their browser? They can then submit whatever they like and your validation code will never run.

Let's take this scenario to the next level. You decide to modify your form to use the OnClick event of your submit button in order to submit the form. At first, this would appear to solve the problem by requiring client-side script execution in order to even get the data to the server, but does it really? Nope... not only have you now upset all your users who don't have client-side scripting enabled, you haven't solved the fundamental problem. There's nothing to stop our crafty hypothetical user from saving the HTML source of your form, modifying it to do whatever they want, and then using their browser to submit it back to your server!

So the lesson here is... use server-side validation for everything that's important. Client-side validation works great as an additional check and to provide clients with a richer web experience, but when it comes down to it you can never really tell what's being sent to your server unless you take the time to look.

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