title: Preventing fake accounts
body: <p>I'm working on a simple web service that allows users to sign up for free and upload a small amount of data. I can easily establish a quota for each user, but malicious users could create fake accounts to upload as much data as they like in a denial-of-service attack.</p>
  
  <p>Obviously, there's no perfect defense against this type of attack, but what can we do to mitigate this problem?</p>