Practical non-image based CAPTCHA approaches?

Question

It looks like we'll be adding CAPTCHA support to Stack Overflow. This is necessary to prevent bots, spammers, and other malicious scripted activity. We only want human beings to post or edit things here!

We'll be using a JavaScript (jQuery) CAPTCHA as a first line of defense:

http://docs.jquery.com/Tutorials:Safer_Contact_Forms_Without_CAPTCHAs

The advantage of this approach is that, for most people, the CAPTCHA won't ever be visible!

However, for people with JavaScript disabled, we still need a fallback and this is where it gets tricky.

I have written a traditional CAPTCHA control for ASP.NET which we can re-use.

However, I'd prefer to go with something textual to avoid the overhead of creating all these images on the server with each request.

I've seen things like..

• ASCII text captcha: \/\/(_)\/\/
• math puzzles: what is 7 minus 3 times 2?
• trivia questions: what tastes better, a toad or a popsicle?

Maybe I'm just tilting at windmills here, but I'd like to have a less resource intensive, non-image based <noscript> compatible CAPTCHA if possible.

Ideas?

Answer 1

CAPTCHA, in its current conceptualization, is broken and often easily bypassed. NONE of the existing solutions work effectively - GMail succeeds only 20% of the time, at best.

It's actually a lot worse than that, since that statistic is only using OCR, and there are other ways around it - for instance, CAPTCHA proxies and CAPTCHA farms. I recently gave a talk on the subject at OWASP, but the ppt is not online yet...

While CAPTCHA cannot provide actual protection in any form, it may be enough for your needs, if what you want is to block casual drive-by trash. But it won't stop even semi-professional spammers.

Typically, for a site with resources of any value to protect, you need a 3-pronged approach:

• Throttle responses from authenticated users only, disallow anonymous posts.
• Minimize (not prevent) the few trash posts from authenticated users - e.g. reputation-based. A human moderator can also help here, but then you have other problems - namely, flooding (or even drowning) the moderator, and some sites prefer the openness...
• Use server-side heuristic logic to identify spam-like behavior, or better non-human-like behavior.

CAPTCHA can help a TINY bit with the second prong, simply because it changes the economics - if the other prongs are in place, it no longer becomes worthwhile to bother breaking through the CAPTCHA (minimal cost, but still a cost) to succeed in such a small amount of spam.

Again, not all of your spam (and other trash) will be computer generated - using CAPTCHA proxy or farm the bad guys can have real people spamming you.

---

CAPTCHA proxy is when they serve your image to users of other sites, e.g. porn, games, etc.

A CAPTCHA farm has many cheap laborers (India, far east, etc) solving them... typically between 2-4$ per 1000 captchas solved. Recently saw a posting for this on Ebay...

Answer 2

So, CAPTCHA is mandatory for all users except moderators. [1]

That's incredibly stupid. So there will be users who can edit any post on the site but not post without CAPTCHA? If you have enough rep to downvote posts, you have enough rep to post without CAPTCHA. Make it higher if you have to. Plus there are plenty of spam detection methods you can employ without image recognition, so that it even for unregistered users it would never be necessary to fill out those god-forsaken CAPTCHA forms.

Answer 3

I personally do not like CAPTCHA it harms usability and does not solve the security issue of making valid users invalid.

I prefer methods of bot detection that you can do server side. Since you have valid users (thanks to OpenID) you can block those who do not "behave", you just need to identify the patterns of a bot and match it to patterns of a typical user and calculate the difference.

Davies, N., Mehdi, Q., Gough, N. : Creating and Visualising an Intelligent NPC using Game Engines and AI Tools http://www.comp.glam.ac.uk/ASMTA2005/Proc/pdf/game-06.pdf

Golle, P., Ducheneaut, N. : Preventing Bots from Playing Online Games <-- ACM Portal

Ducheneaut, N., Moore, R. : The Social Side of Gaming: A Study of Interaction Patterns in a Massively Multiplayer Online Game

Sure most of these references point to video game bot detection, but that is because that was what the topic of our group's paper titled Robot Wars: An In-Game Exploration of Robot Identification. It was not published or anything, just something for a school project. I can email if you are interested. The fact is though that even if it is based on video game bot detection, you can generalize it to the web because there is a user attached to patterns of usage.

I do agree with MusiGenesis 's method of this approach because it is what I use on my website and it does work decently well. The invisible CAPTCHA process is a decent way of blocking most scripts, but that still does not prevent a script writer from reverse engineering your method and "faking" the values you are looking for in javascript.

I will say the best method is to 1) establish a user so that you can block when they are bad, 2) identify an algorithm that detects typical patterns vs. non-typical patterns of website usage and 3) block that user accordingly.

Answer 4

Recently, I started adding a tag with the name and id set to "message". I set it to hidden with CSS (display:none). Spam bots see it, fill it in and submit the form. Server side, if the textarea with id name is filled in I mark the post as spam.

Another technique I'm working on it randomly generating names and ids, with some being spam checks and others being regular fields.

This works very well for me, and I've yet to receive any successful spam. However, I get far fewer visitors to my sites :)

Answer 5

What about a honeypot captcha?

Answer 6

I know that no one will read this, but what about the dog or cat CAPTCHA?

You need to say which one is a cat or a dog, machines can't do this.. http://research.microsoft.com/asirra/

Is a cool one..