More from the mailbag - user name lockouts

  • Article

Dan asked (a long time ago, sorry Dan) that he was getting notifications of accounts being locked out due to invalid password attempts. Dan's assuming this is because someone is trying to hack into an account and wanted to know how to stop SBS usernames from being "broadcast".

Unfortunately, the question makes a fundamental assumption, that SBS somehow is broadcasting usernames. This is not the case, but it probably doesn't take someone long to figure out a username for your users.

For example, if your user's e-mail addresses and logon names are identical, then once they get an e-mail address for a user they can simply use that as their username for their attempt and start a dictionary attack.

How does one get a valid e-mail address? Simple: start at a@foo.com, end at zzzzzzzz@foo.com, and try every combination in between. Run a script to send to your SBS server a piece of spam that will just get deleted by your ordinary user. Wait for NDRs to return for the non-valid addresses, subtract the rest, and you have a user list.

2 ways to obscure this information:

  1. Make your user's logon name and e-mail address different (can be done in the Add User Wizard at user creation time or afterwards by modifying the e-mail address or logon name in the user properties)
  2. Disable external deliveries of NDRs (840158 You cannot restrict certain automatic responses to the Internet based on http://support.microsoft.com/?id=840158) Note that I don't typically recommend this as it is technically a violation of the SMTP RFC and won't help your customers who accidentally mis-type an e-mail address and wonder why they never got a response

Your most effective way of making sure that no one cracks your accounts is to enforce strong password policy. In Dan's case, though, it doesn't help his users who are getting locked out of their account legitamately. In that case, your best effort is to take some time and try to determine the IP address of the offending attempts and then block these at your router or higher up.