A brute-force attack (aka BF-attack) is an attack on the system by probing of all possible combinations of inputs to the login forms (user names, passwords, and any other information there).
If a system is able to tell humans from computers apart, it can defend itself from mechanical guesses.
A CAPTCHA is a challenge that humans can solve easily, while computers have difficulties with. Thus a CAPTCHA serves as an effective, (though not completely bullet-proof) mechanism for protecting a system from brute-force attacks.
When a certain number of incorrect login attempts occur, the Survey Solutions login form will start asking a CAPTCHA challenge:
The above shown CAPTCHA shows if the server administrator has configured Survey Solutions to utilize Google’s captcha service. If this service is not used, Survey Solutions will rely on its built in CAPTCHA, which appears like the following:
If the user continues to enter incorrect combinations of login/password even with the CAPTCHA, an autolock will eventually engage.