[userbinator]

What are the "bad guys" going to do, create accounts and submit problem solutions...? There doesn't appear to be anything worth "attacking" here anyway, so I don't think this is a "security problem" at all.

[joshdick]

The captcha was introduced to Project Euler a few years ago in response to someone spamming the site with requests to check an answer.

Apparently, someone had trouble with a question, so they wrote a script to submit every possible number.