Less than two months ago, Apple users discovered a bug in macOS that allowed anyone to log in with root access. Apple apologized and fixed it quickly, but now users on Open Radar have found a similar (but far less severe) macOS password bug.
If you're running macOS High Sierra, try this:
- Open System Preferences.
- Click on App Store.
- If the padlock is unlocked, click to lock it.
- Click the padlock to unlock it.
- In the prompt, enter your username and any password.
The App Store preferences pane should unlock. We tried it on a new iMac and MacBook Pro, both with macOS 10.13.2, and it worked.