Question

I’m putting together a small web app that writes to a database (Perl CGI & MySQL). The CGI script takes some info from a form and writes it to a database. I notice, however, that if I hit ‘Reload’ or ‘Back’ on the web browser, it’ll write the data to the database again. I don’t want this.

What is the best way to protect against the data being re-written in this case?

Answer 1

Do not use GET requests to make modifications! Be RESTful; use POST (or PUT) instead the browser should warn the user not to reload the request. Redirecting (using HTTP redirection) to a receipt page using a normal GET request after a POST/PUT request will make it possible to refresh the page without getting warned about resubmitting.

EDIT:

I assume the user is logged in somehow, and therefore you allready have some way of tracking the user, e.g. session or similar.

You could make a timestamp (or a random hash etc..) when displaying the form storing it both as a hidden field (just besides the anti Cross-Site Request token I’m sure you allready have there), and in a session variable (wich is stored safely on your server), when you recieve a the POST/PUT request for this form, you check that the timestamp is the same as the one in session. If it is, you set the timestamp in the session to something variable and hard to guess (timestamp concatenated with some secret string for instance) then you can save the form data. If someone repeats the request now you won’t find the same value in the session variable and deny the request.

The problem with doing this is that the form is invalid if the user clicks back to change something, and it might be a bit to harsh, unless it’s money you’re updating. So if you have problems with ‘stupid’ users who refresh and click the back-button thus accidentally reposting something, just using POST would remind them not to do that, and redirecting will make it less likely. If you have a problem with malicious users, you should use a timestampt too allthough it will confuse users sometimes, allthough if users is deliberately posting the same message over and over you probably need to find a way to ban them. Using POST, having a timestam, and even doing a full comparison of the whole database to check for duplicate posts, won’t help at all if the malicious users just write a script to load the form and submit random garbage, automatically. (But cross-site-request protection makes that a lot harder)