Tuesday, December 30, 2014

That CAPTCHA moved!

While recovering a password for a site -- that is, my real password was whatever information the recovery page needed -- I noticed a new wrinkle on CAPTCHA: Moving CAPTCHA.  Instead of the usual smeared-out or obscured letters, three plainly readable letters, somewhat tilted, on a clearly contrasting background, but wiggling slightly back and forth.

Seems like an interesting step in the whole OCR arms race, except ...

The problem for an attacker to solve here isn't recognizing a moving character, which might or might not be harder than recognizing a still one.  It's grabbing a frame of the animation to examine.  If you can do that at all, then recognizing one particular arrangement of the letters is no harder than recognizing any other CAPTCHA.  Easier, in fact, since you have nice, legible letters, and you can re-run the OCR on each frame and go with the consensus.

Again, I haven't looked at this in detail, but there would seem to be two main ways of putting the moving image up in the first place: A .gif or other animated image format, which is no problem to decode into its images, or some sort of JavaScript animation.  That might be harder to grab, but not because of the animation.  You can just as well use JavaScript to put up a still image, and in either case the answer is to render the JavaScript and then grab the pixels.

In other words, it seems unlikely that the moving image adds any real difficulty for an attacker.  It does look harder, intuitively, to the human eye, but the attacker isn't using a human eye -- that's the whole point of the exercise to begin with.

No comments: