Go to a web site that requires a login. Put in any username with any password. Did the page come back with both the User and Password fields blank? Now put YOUR username in, but with some password you make up. Does the form come back with your username in the User field and nothing in the Password field? If so, here’s what you just discovered. The developer is making his form more efficient by not hashing and testing the password to see if it’s correct unless the username is valid. If the username IS valid, he populates the User field with it and checks the password. If the password is incorrect, he only clears the Password field so you can retry your password. You just discovered a crude form of username harvesting. Try different usernames and if they remain in the User field, that’s a valid account on the server. I know, that would take a lot of time to do it that way. That’s why hackers write automated tools.
*** This is a Security Bloggers Network syndicated blog from JeffSoh on NetSec authored by JeffSoh. Read the original post at: http://jeffsoh.blogspot.com/2017/06/simple-username-harvesting-from-sans.html