Recently I had an elderly client that could not remember the password to his iMac nor the email passwords configured in Apple Mail. I was caught in a situation where I could not reset his Keychain as that would remove his email passwords, but I also had no way of extracting passwords from his Keychain. On top of that I needed the password to install new printer drivers.
This information is intended to support others that have forgotten their login password, it should not be used for evil. If you want to secure your system from vulnerabilities like this it is important to enable FileVault whole-disk encryption and use a unique/secure password. You can turn it on under System Preferences > Security & Privacy > FileVault.
I remembered reading how the Automatic Login feature stores a cipher of the user's password in /etc/kcpassword. A quick Google search later and I had the following command to extract the password.
Since the above command requires sudo and I didn't have access to an administrator account, I booted the iMac into Target Disk Mode, connected it to another Mac (via FireWire/Thunderbolt cable) and ran the following command:
sudo ruby -e 'key = [125, 137, 82, 35, 210, 188, 221, 234, 163, 185, 31]; IO.read("/Volumes/Macintosh HD 1/etc/kcpassword").bytes.each_with_index { |b, i| break if key.include?(b); print [b ^ key[i % key.size]].pack("U*") }'And just like that I had recovered his password!