Security Problem : In any wordpress version it is possible to get a valid loginname by trial and error by the use of the login screen.  What happens that in case you enter a wrong username that username is removed after the login attempt. However if you enter a correct username that username stays visible in the login screen.

Both attempts also generate a different error message explaining that you either provided the wrong username and or password or the wrong password. In the latter case wordpress indirectly is telling you that you entered the valid and correct username […]