I mean can I login using Advanced options -> Recovery Menu as root user without any restriction (except for a Full Disk Encrypted system)? Can someone explain how to fix this security hole?

Visual explanation: screenshot of recovery menu

marked as duplicate by Byte Commander, user535733, user68186, muru, wjandrea Nov 14 '18 at 23:22

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    It's not a security hole - it's deliberate. Using that option requires physical access to the machine, which means all non-encryption forms of security can already be bypassed. If it bothers you, then use encryption. – user535733 Nov 14 '18 at 22:33
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    Also have a look at askubuntu.com/q/676545/367990 – Byte Commander Nov 14 '18 at 22:38
  • It is a bit not obvious when we installing operating system and it asks to create a secure-good password for user and open doors to any with most full privileges. – mature Nov 14 '18 at 22:40
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    Physical security has always been just as important as account security. Now you know. Spread the word so your friends don't make the same mistake. – user535733 Nov 14 '18 at 22:44
  • SSH root access isn't looks like is Physical. – mature Nov 14 '18 at 22:54