Last modified: 2013-11-24 20:04:23 UTC
This was only discovered by creating hundreds of accounts (one for each wiki), and is therefore perhaps not a real security concern.
There are sometimes repeated CAPTCHAs or words which shouldn't appear. Once a CAPTCHA is used, it should likely be binned for all wikis. On several occasions I've had the same CAPTCHA more than once.
The only example I can remember is alsopoet, which has been used two times, but there are others (I mentioned this previous to Brion in IRC).
Yeah, the current scheme is not very super and will be biased to some items depending on the existing random distribution. A nice queue with proper expiration and refreshing would be better.
There are 10,000 captchas, so coincidences are likely after ~sqrt(10000) = 100 attempts. That's not a break in itself, the break comes if the attacker is able to manually solve say 100 captchas, and then do a brute force attack with a success rate of 1%. But an OCR method may well give a better hit rate, so it might not be worth all that human effort to build the dictionary. A manually-constructed dictionary can easily be invalidated by regenerating the captchas, but an OCR method would require a change to the algorithm.
Changed title, the problem is the challenge set size, not the randomness. It's perfectly random.
Not a MediaWiki, but a Wikimedia issue in FancyCaptcha images generation, and possibly a superseded one if Aaron produced the last set with a bigger dictionary. Aaron?
Aaron: Could you answer comment 4 please?
There is an option in confirmedit ($wgCaptchaDeleteOnSolve) to delete the captchas after they are used. See bug 24730.