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I am a system administrator and I do not want users to use su - $appuser and try entering the password of the app user

rather

use sudo su - $appuser and enter their own password.

Some naïve users who are not aware try to do su - $appuser and enter an incorrect password, by doing this the application user gets locked after 3 unsuccessful attempts (defined by the AD team; cannot be changed). We want to force users to use sudo su -

Can this be achieved ?

  • Which OS is this? – muru Jan 17 '17 at 4:20
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    Why sudo su? It's a horrible construct and totally unnecessary. – roaima Jan 17 '17 at 20:31
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Well, you could always use the traditional BSD-route...

Create a group - wheel is the traditional name...

Add users allowed to use su into this group - in your case, none (maybe except root, but root is allowed anyway).

Set the group-ownership of su to wheel

Take away others execution-rights for su

By the end, su should have root:wheel ownership, and it's permission should be: rws--x--- (ie. chmod u=rwxs,g=x,o= su) (or even better; --s--x--- ; ie. chmod u=xs,g=x,o= su)

This will prevent anybody but root and members of wheel from executing su. Since using sudo effectively makes any user root, sudo su ... should also work.

  • There isn't really much gained from removing a+r on /bin/su. At least if it's the one your distro ships—anyone can just download it from the Internet. – derobert Jan 18 '17 at 22:03
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If you're on a system that uses PAM (e.g., almost all Linux systems), then you have a few more options.

  1. Edit /etc/pam.d/su and add a line like su auth required pam_wheel.so before your LDAP/AD/etc. authentication. This gives you the wheel group, through PAM instead of file ownership. You can also do much more flexible things using pam_succeed_if.
  2. Edit the same, and either change auth sufficient pam_rootok.so (which lets root use su) to auth required pam_rootok.so (to let only root use su), or alternatively put auth required pam_deny.so at some point.
  3. Same file, you could do something like auth required pam_succeed_if.so uid ne $appuser to just block su to the app user.

I apologize in advance I haven't tested any of those.

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