Best Captcha-Solving Service

Yep, and if you use Solve Media you should skip the ads! The reason is that there are two easy ways to attack this captcha:

1) Solve Media is basically an ad network. As with all ad networks, the number of ads in their inventory at any one time is going to be limited (how many times do you refresh a page and get the same ad?). An attacker could simply collect the ads together with their answers and do a replay attack.

2) Even if Solve Media modified their systems so that the ads are shown with some differences (i.e, some noise etc) a machine vision attack can be implemented similar to what was done to break the ASIRRA captcha.

BTW: ASIRRA is broken and broken good! Golle (2008) described a Machine Vision attack against ASIRRA with a 10% probability of breaking the captcha (a captcha is supposed to stop 9,999 attempts out of every 10,000, so 1 in 10 is horrendous and renders the captcha useless).

I put together a Machine Vision attack against ASIRRA (only theoretical, of course) that used an ensemble of Bayesian classifiers to solve ASIRRA captchas with a 43% success rate. The rate held up even if the images were corrupted with 25% noise (23% success rate). I have not tried this on the Solve Media captchas, though, but I think it would be an interesting exercise :rolleyes:

References

Golle, P. (2008). Machine Learning Attacks Against the ASIRRA CAPTCHA. In Proc. CCS 2008, ACM Press (2008), 535-542.