What Is Username Enumeration Prevention
By default Drupal is very secure (especially Drupal 7). However, there is a way to exploit the system by using a technique called username enumeration. Both Drupal 6 and 7 have this issue, but it is much worse for people using Drupal 6. This is because Drupal 6 does not have any built in brute force prevention. When an attacker knows a username they can start a brute force attack to gain access with that user. To help prevent this, it is best if usernames on the system are not easy to find out.
Attackers can easily find usernames that exist by using the forgot password form and a technique called “username enumeration”. The attacker can enter a username that does not exist and they will get a response from Drupal saying so. All the attacker needs to do is keep trying usernames on this form until they find a valid user.
This module will stop this from happening. When the module is enabled, the error message will be replaced for the same message as a valid user and they will be redirected back to the login form. If the user does not exist, no password reset email will be sent, but the attacker will not know this is the case.
For a demonstration of this, visit the password reset page at zeusarticles.com and try to enter an invalid username. This will show you the same message as a valid username.