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I did the following steps.

  1. mv ~/.local/share/keyrings/login.keyring ~/.local/share/keyrings/login.keyring.bakup
  2. Reboot computer
  3. Choose advanced options -> recovery -> boot as root

This got me to root terminal. Password was not required.

Now I can do whatever I want, e.i. format the disk. Or I can do better. I've created a new unwanted_user and added it to sudoers group. Moved the keyring from login.keyring.bakup to login.keyring, while keeping permissions. Rebooted Ubuntu one more time. I could have installed SSH-server, connect to this computer from a remote location, do my stuff, cleanup logs and delete the unwanted_user.

Sure, an admin in a government security office would lock his computer, notice that the computer is now at a login screen, check logs, have surveillance cameras, etc.

But what if I am a layman, hiding my private documents in a folder, owned by root? Anyone familiar with Linux can repeat these steps. Is this a feature or a bug. Can I disable this unwanted keyring service? I've uninstalled the seahorse, but the same steps worked again.

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This got me to root terminal. Password was not required.

...

But what if I am a layman, hiding my private documents in a folder, owned by root? Anyone familiar with Linux can repeat these steps. Is this a feature or a bug.

If you have physical access to a computer you can always find a way to access (and change) unencrypted data on it. This isn't really related to the login keyring, once you boot in the single user mode, you are root and you can do whatever you want, you don't need to do anything with the login keyring. You can add password protection to GRUB to make sure your system can't be booted in a single user mode, but simply booting from a Live USB will give you the same full access.

If you want to hide your files, don't hide them in a folder owned by root, encrypt your disk and if other people have access to your computer, never leave it without locking the screen or shutting it down.

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  • Well, then why do we bother with permissions and user privileges if a simple advanced boot settings -> boot as a root grants you root access to the computer? You cannot alter system files on Windows 10 without elevating the privilege (with password).
    – sixtytrees
    Commented Jul 23, 2021 at 11:25
  • ...Will give you the same root access if you can physically connect the harddrive to another computer.... In Windows I can store my computer in a locked case with no access to plug LiveOS. In Ubuntu you can.
    – sixtytrees
    Commented Jul 23, 2021 at 11:27
  • Got it, I should add a password protection of GRUB, which is off by default. Thank you, answer accepted.
    – sixtytrees
    Commented Jul 23, 2021 at 11:28
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    @sixtytrees the main point is what Vojtech says in the first paragraph: if you have physical access to the computer, then the computer is considered compromised. For example, I can simply remove the hard drive, put it into my computer where I have root access and do whatever I want. There is no security that can protect from an attacker with physical access to the machine. All you can do is make it slightly harder.
    – terdon
    Commented Jul 23, 2021 at 11:56
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    If they're encrypted, yes. But a capable geek knows how to boot a machine from a USB live OS image, @sixtytrees, so will be able to boot your computer under a different OS and access all your data. If you want protection from a minimally capable geek on any operating system, you should encrypt.
    – terdon
    Commented Jul 26, 2021 at 21:57

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