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I want to prevent users with sudoer access from using sudo to execute passwd (and thus changing the root password).

Is this possible? If so, how can it be accomplished?

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Anyway, why is so important to have the root password when you are already a sudoer? The lecture about that "with great power comes great responsability" that sudo gives at first run is exactly the answer to this question. That or either give "restricted" sudo permissions (only certain programs, and assure those can't execute subshells, so no more or less) – ssice Jan 3 '15 at 3:09

You basically cannot forbid users from executing a particular command, if they're still allowed to execute all other commands – they could copy passwd to really-not-passwd-i-promise and execute that.

If you want to stop sudoers from changing root's password – or somebody else's password – you cannot prevent that either; they can manually edit /etc/shadow as long as they have root privileges using sudo.

(This could be achieved by e.g. using Kerberos or LDAP for authentication – that way nobody could change another user's password – but still nothing can prevent sudoers from simply breaking configuration or something like that.)

Basically, don't give full sudo rights to people you do not trust. Forbidding a single command and allowing all others cannot be done.

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+1 - The only option I could think of to allow some, but not all, sudo rights would be to create a user group with access to some of the root abilities. But @PaulDC, there should be little reason why other users outside of the sysadmin should have root privs (especially questionable users). Do some research for user groups and visudo. It may get you the closest possible (but you need to define each specific thing that can be done to limit it, and avoid editors like vim/emacs since that would allow file editing with admin rights). – nerdwaller Apr 5 '13 at 3:03
    
Editing files, spawning subshells, assigning security contexts and setting setuid/capabilities should be specially taken care of. But if you can't do any of that, why would you need to impersonate root anyway? Maybe you would like to further research into ACLs, unix groups and/or sudo as users other than root. – ssice Jan 3 '15 at 3:13

It's very difficult to keep root user out of anything, but I believe you can do it with AppArmor.

The problem is, how would you set it up so that legitimate password changes can happen?

The trick might be to set up an AppArmor profile for sudo that is inherited by all its children. This wouldn't affect users running passwd as themselves (to change their own password) and wouldn't affect users who enter root via su.

However, I suspect you could circumvent that by copying, moving or renaming the sudo binary, so you'd probably want to lock down passwd write-access for all root processes, and then only ever change it using a live CD.

Of course, you also have to protect AppArmor itself from modification. :)

The best solution is probably too give sudo access only for specific commands and then only for commands that don't change the filesystem.

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