In one embodiment, accessing a first image of a CAPTCHA, wherein the CAPTCHA comprises: the first image; a challenge based on the first image; and a correct response based on the first image; constructing a second image by scrambling the first image, comprising: cutting the first image into a plurality of image sections; recording positions of the image sections in the first image; and randomly rearranging the image sections to construct the second image; generating web-browser-executable code for unscrambling the second image back to the first image, wherein the web-browser-executable code indicates, for each one of the image sections in the second image, the position of the image section in the first image; and transmitting the second image and the web-browser-executable code to a second computing device.