The Unsecured Desktop
During the initial installation of Ubuntu, the setup wizard asks if you want to require a password to log in, or if you want the system to log you in automatically. If you chose the automatic option (or if a system administrator set up your machine this way), Ubuntu completely bypasses the login screen upon boot. The moment you turn on the computer, it drops you straight onto the GNOME desktop, logged into your specific user account. While incredibly convenient for a media center PC in your living room, this is a massive security risk for a laptop that could be lost or stolen, as anyone opening the lid instantly gains full access to your files and saved browser passwords.
Enforcing Password Authentication
To secure your data and stop Ubuntu from automatically logging in on boot, you must disable the automatic login feature in the user management settings.
Open the main Ubuntu application launcher (press the Super/Windows key) and type Settings. Click the icon to open the main configuration panel. Scroll all the way down the left-hand navigation sidebar and click on System, then click on Users. You will see your profile listed at the top. First, you must click the Unlock button in the top right corner (or next to your name) and enter your current administrator password to authorize changes. Once unlocked, look for the toggle switch explicitly labelled Automatic Login. Click this toggle to turn it Off. Close the window. The next time you reboot or wake the computer from sleep, Ubuntu will halt at the lock screen and demand your password.