ASP.NET MVC: Is Data Annotation Validation Enough?

Question

I'm using the Data Annotation validation extensively in ASP.NET MVC 2. This new feature has been a huge time saver, as I'm now able to define both client-side validation and server-side validation in one place. However, while I was doing some detailed testing, I realized that it's quite easy for someone to bypass the server-side validation if I relied on Data Annotation validation alone. For example, if I defined a required field by annotating the property with the [Required] attribute and placed a textbox for that required field in a form, a user could simply remove the textbox from the DOM (which can easily be done through Firebug) and now the Data Annotation validation will not be triggered on that property during ModelBinding inside of a Controller. To ensure that the "required" validation is triggered, I can repeat the validation after ModelBinding happens, but then I'd be repeating my validation logic.

What is everyone's recommendation on validation? Is Data Annotation validation enough? Or does the validation need to be repeated to ensure that validations get triggered in all situations?

Follow-up comment: Based on the answers below, it seems that I can't rely on the Model Binder and Data Annotation validation alone. Since we're concluding that additional server-side validation is required, is there an easy way for my Service layer to trigger validation based on what's been defined in the Data Annotations? It seems that this will get us the best of both words...we won't need to repeat the validation code, but we'll still ensure that the validation gets executed even if Model Binder doesn't trigger it.

I'm going to post this follow-up comment as a separate question, as it poses a different question than the original one.

Answer 1

I think to be vigilant concerning security you should choose to you make server validation the priority and ensure that this is always your fallback. Your server validation should work without the client validation. Client validation is more for UX and tho that is paramount to your design, it is secondary to security. With this in mind you will find yourself repeating your validation. A goal is often trying to design your app so that the server and client validation can be integrated as much as possible to reduce the work required to validate on the server and the client. But be assured you must do both.

If bypassing the client validation (by means of DOM manipulation) is avoiding the server validation (which it seems you are indicating) then your server validation for this instance may not be employed appropriately. You should be invoking your server validation again in your controller action or in a service layer. The scenario you describe should not be defeating your server validation.

With the scenario you describe, the DataAnnotation attributes method should be sufficient. It seems that you simply need to make a few code changes to ensure that your server validation is invoked also when submitting the form.