I am making a small SAAS application. I am using the same database for all users and only differentiating between the data using username and ids. This means that the user can type in a new url in the browser and see other users data. This, of course is not a desirable approach. I would like to do a check to see if the current user can actually access the resources: eg.
http://myapplication.com/images/15
And if the user changes the url to
http://myapplication.com/images/16
I should do a check in my database to see if the current user actually has access to see the images with user id 16. And if not redirect to a "not authorized" page.
How do I implement this?
The first step is to make sure that you never have any ID's for the user itself in the url. For instance, never have http://example.com/?user=10. You should always get the users id from their authentication rather than from the URL (or posted values either).
The second step, is to use that ID in your queries. So, for instance, let's say they seek http://example.com/images/100, then in your database you should have a mechanism that links the asset's ownership to the user, either a userid or a mapping table of id's to asset's, etc.. This way, if the user isn't allowed access, it will just return an empty result set. It's impossible for the data to be returned, and the empty result set should tell your page that the item doesn't exist (not necessarily an authorization failure, just that the object doesn't exist).
Third, any pages which are inherently about the user, such as a user profile, account page, or dashboard should never have any ID's at all in the URL, it should just automatically go to the authenticated users page.
Finally, if you need to prevent the user from accessing an entire page or set of pages, then you should do this in the OnAuthorization event or similar (custom attribute, base class, etc..) or using the built-in attribute authorization and use role based authorization. Never do authorization in the PageLoad or similar event (such as the controller action), because by the time you get to that step a lot of work has already happened in the pipeline. It's best to block access long before the page even starts to setup. Authorization events happen at the very beginning of the pipeline.