Stop spam without captcha

Question

I want to stop spammers from using my site. But I find CAPTCHA very annoying. I am not just talking about the "type the text" type, but anything that requires the user to waste his time to prove himself human.

What can I do here?

Answer 1

Integrate the Akismet API to automatically filter your users' posts.

Answer 2

Requiring Javascript to post data blocks a fair amount of spam bots while not interfering with most users.

You can also use an nifty trick:

<input type="text" id="not_human" name="name" />
<input type="text" name="actual_name" />
<style>
   #not_human { display: none }
</style>

Most bots will populate the first field, so you can block them.

Answer 3

I would be careful using CSS or Javascript tricks to ensure a user is a genuine real life human, as you could be introducing accessibility issues, cross browser issues, etc. Not to mention spam bots can be fairly sophisticated, so employing cute little CSS display tricks may not even work anyway.

I would look into Akismet.

Also, you can be creative in the way you validate user data. For example, let's say you have a registration form that requires a user email and address. You can be fairly hardcore in how you validate the email address, even going so far as to ensure the domain is actually set up to receive mail, and that there is a mailbox on that domain that matches what was provided. You could also use Google Maps API to try and geolocate an address and ensure it's valid.

To take this even further, you could implement "hard" and "soft" validation errors. If the mail address doesn't match a regex validation string, then that's a hard fail. Not being able to check the DNS records of the domain to ensure it accepts mail, or that the mailbox exists, is a "soft" fail. When you encounter a soft fail, you could then ask for CAPTCHA validation. This would hopefully reduce the amount of times you'd have to push for CAPTCHA verification, because if you're getting enough activity on the site, valid people should be entering valid data at least some of the time!

Answer 4

If you're looking for a .NET solution, the Ajax Control Toolkit has a control named NoBot.

NoBot is a control that attempts to provide CAPTCHA-like bot/spam prevention without requiring any user interaction. NoBot has the benefit of being completely invisible. NoBot is probably most relevant for low-traffic sites where blog/comment spam is a problem and 100% effectiveness is not required.

NoBot employs a few different anti-bot techniques:

• Forcing the client's browser to perform a configurable JavaScript calculation and verifying the result as part of the postback. (Ex: the calculation may be a simple numeric one, or may also involve the DOM for added assurance that a browser is involved)
• Enforcing a configurable delay between when a form is requested and when it can be posted back. (Ex: a human is unlikely to complete a form in less than two seconds)
• Enforcing a configurable limit to the number of acceptable requests per IP address per unit of time. (Ex: a human is unlikely to submit the same form more than five times in one minute)

More discussion and demonstration at this blogpost by Jacques-Louis Chereau on NoBot.

<ajaxToolkit:NoBot
  ID="NoBot2"
  runat="server"
  OnGenerateChallengeAndResponse="CustomChallengeResponse"
  ResponseMinimumDelaySeconds="2"
  CutoffWindowSeconds="60"
  CutoffMaximumInstances="5" />