User account names are often highly structured (e.g. Joe Bloggs account name is jbloggs and Fred Nurks account name is fnurks) and valid account names can easily be guessed.
Determine whether a consistent account name structure renders the application vulnerable to account enumeration. Determine whether the application's error messages permit account enumeration.
Ensure the application returns consistent generic error messages in response to invalid account name, password or other user credentials entered during the log in process.