It's time to rethink some old, and now outdated, security truisms that enable a very scary kind of attack.
As computing power and programming prowess have increased, so have the means to make this attack simple, effective and lightning-quick, to the point where it's now a huge risk. Tools are freely available to let someone own your entire Active Directory infrastructure in a matter of minutes, without brute-force cracking or any other compute-intensive resources.
The attack is not new: It's known as a "passing-the-hash" attack, and it's been around for years. Those proficient in security matters know of the attack and the general principle behind it: Passwords are converted by Windows into a "hash" -- a fixed-size string of cryptographically transformed data -- every time they are created. This way, your passwords aren't sitting in plain text anywhere on your machine, nor are they transmitted in clear text when the authentication happens over the wire.