Upgrade your computer now...
A previously unknown security bug in macOS High Sierra has now been patched by Apple through Security Update 2017-001. If you use an Apple computer running macOS High Sierra, we recommend that you upgrade your OS immediately.
The recently discovered bug allows attackers may to bypass administrator authentication without supplying the administrator’s password. This makes it extremely simple for third parties to break into almost any computer running macOS High Sierra.
To update your OS:
More information: Apple