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I locked my screen for a moment and now I am unable to login back into my Ubuntu 14.04 machine, not even in a "Guest Session". After a moment of panic, I realized that I am the responsible, as I think I executed the following (I'm 99% sure) a moment ago: 

sudo chmod 555 /etc/shadow

Is there a way I can restore the original permissions of /etc/shadow (with an Ubuntu live CD maybe?). I'm also 99% sure no other files were modified in my moment of stupidity.

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The standard permissions for the file /etc/shadow are 640 (-rw-r-----)

% stat -c "%a %n" /etc/shadow
640 /etc/shadow

% ls -la /etc/shadow
-rw-r----- 1 root shadow 1870 Aug 25 17:32 /etc/shadow

Therefore boot your system in recovery mode (you need only the steps 1..4) and change the permission again:

chmod 640 /etc/shadow

In the recovery mode, you don't need sudo

If that does not work, because you have to enter a password and it does not work, follow Plan B:

  • Boot a Live System
  • Mount the root / partition, usually /etc is not located in its own partition

  • Go into the roou account

    sudo su

  • Change the permission

    chmod 640 /etc/shadow
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  • When I open the root console, I am prompted Give root password for maintenance and I get stuck at that point.
    – Яois
    Aug 28, 2015 at 7:23
  • Plan B, boot a Live System, mount the partition and change the permission.
    – A.B.
    Aug 28, 2015 at 7:47
  • I used plan B and it worked. It was worse than I thought, as I had modified the content of /etc/shadow, so what I did was re-generating it from /etc/passwd using pwconv. It didn't even work after that though, so I passwd to generate a new root and user passwords and everything is back to normal now.. hopefully :)
    – Яois
    Aug 28, 2015 at 8:45

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