Defeating reddit’s CAPTCHA

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Here’s something we’re sure SEO specialists, PR reps, and other marketeers already know: how to write a script to game reddit.

The course of upvotes and downvotes controls which submission makes it to the front page of reddit. These submissions are voted on by users, and new accounts must log in and complete a CAPTCHA to vote. [Ian] discovered that reddit’s CAPTCHA is not really state-of-the-art, and figured out how to get a bot to solve it

The method exploits the 8-bit nature of the distorted grid in the CAPTCHA. Because this grid isn’t pure black or pure white, it’s at a lower intensity than the letters in the CAPTCHA. Putting the CAPTCHA through a threshold filter, deleting any blocks of pixels smaller than 20 pixels, and running it through a classifier (PDF there), a bot can guess what the letters of the CAPTCHA should be.

Out of the 489 CAPTCHAs [Ian] fed into his algorithm, only 28 – or 5.73% – were guessed correctly. However, because he knows which CAPTCHAs had failed segmentation, ignoring those can increase the success rate to 10%. Theoretically, by requesting new CAPTCHAs, [Ian] can get the accuracy of his CAPTCHA bot up to about 30%.

Combine this with a brilliant auto voting script that only requires someone to enter CAPTCHAs, and you’ve got the recipe for getting anything you want directly to the front page of reddit. Of course you could do the same with a few memes and pictures of cats, but you knew that already.

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