Preventing Brute Force Logins on Websites

Question

As a response to the recent Twitter hijackings and Jeff's post on Dictionary Attacks, what is the best way to secure your website against brute force login attacks?

Jeff's post suggests putting in an increasing delay for each attempted login, and a suggestion in the comments is to add a captcha after the 2nd failed attempt.

Both these seem like good ideas, but how do you know what "attempt number" it is? You can't rely on a session ID (because an attacker could change it each time) or an IP address (better, but vulnerable to botnets). Simply logging it against the username could, using the delay method, lock out a legitimate user (or at least make the login process very slow for them).

Thoughts? Suggestions?

Answer 1

I think database-persisted short lockout period for the given account (1-5 minutes) is the only way to handle this. Each userid in your database contains a timeOfLastFailedLogin and numberOfFailedAttempts. When numbeOfFailedAttempts > X you lockout for some minutes.

This means you're locking the userid in question for some time, but not permanently. It also means you're updating the database for each login attempt (unless it is locked, of course), which may be causing other problems.

There is at least one whole country is NAT'ed in asia, so IP's cannot be used for anything.