Exam 312-50v12 topic 1 question 270 discussion

Calvin, a grey-hat hacker, targets a web application that has design flaws in its authentication mechanism. He enumerates usernames from the login form of the web application, which requests users to feed data and specifies the incorrect field in case of invalid credentials. Later, Calvin uses this information to perform social engineering. Which of the following design flaws in the authentication mechanism is exploited by Calvin?