Bcrypt slows down a brute force attack considerably, but cannot prevent one. If your password is one of the top N passwords in use, then a determined attacker

Bcrypt slows down a brute force attack considerably, but cannot prevent one. If your password is one of the top N passwords in use, then a determined attacker can still break it. For example, if you set your work factor such that it takes 1 second to verify a password using bcrypt on login to your server, then if your password is in a dictionary of the top 100k passwords, an attacker could get to it within (about) a day using equivalent hardware. Using more specialized (or just more) hardware of course drives it further.