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Spam Prevention without CAPTCHA Images

The ASP.NET AJAX Control Toolkit (formally known as AJAX) offers a control that reduces spam on web logs and forums without requiring users to enter a CAPTCHA.

The NoBot control uses three techniques to detect bots.

  • First, it offers a framework for a challenge/response in the form of a client-side JavaScript calculation. This will filter out bots that don't support JavaScript.
  • Second is a minimum delay. The idea behind this is if you know that users cannot possible fill out a form in less than 2 seconds, you can assume they are bots.
  • Finally it can limit the number of submissions from a given IP address in a time period. For example, a human isn't likely to make 5 posts in a single minute.

While these techniques are not as effective as an image based CAPTCHA, they are more user friendly and don't require accessibility fall backs for the visually impaired.

The October release of the ASP.NET AJAX Control Toolkit can be downloaded from CodePlex.

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