[bdc]

Actually, the captcha predicate is a GET request that looks like this:

http://en.lichess.org/captcha/78pkfjgz?solution=e5+e4 which returns 1 or 0.

So the brute force captcha solution is to go through 8^4 (~4k) get requests, which you could code in five minutes and execute in a minute.

[MichaelGG]

I would imagine it'd be fairly trivial to limit the number of requests.