Vrftjkry

I've seen this question asked before here and here but they're pretty old and I haven't been able to get any solution to work.

Basically I need to run a script or command automatically as root each time a non root user SSH's into the server. Server is running Ubuntu 16.04 and using OpenSSH if that makes a difference.

EDIT:
To be more specific, I want to kill a process that was created by user A when user B logs in. My roommates and I mine crypto on my headless gaming server and I'd like to be able to run pkill miner.sh automatically when someone logs in, regardless of who started it. Since it would be insanely insecure to let users kill each others' processes, it seems that this is more difficult than expected.

pam_exec has you pointed in the right direction, but setuid is bad advice; most (all?) modern Linuxes ignore it on shell scripts and only respect it on binary executable files.

Would it be a problem if the script was executed (as root) at times other than login?
If it's ok for it to run more often than strictly required, you could create a wrapper script that's executed at every ssh login with user permissions which uses sudo (with no password) to execute your script as root.
This, however, means that the users could also run it manually (as root) as often as they wanted to.

To set this up, add this line to /etc/pam.d/sshd

account optional pam_exec.so /etc/pam.d/LoginWrapper.sh

Then create the file /etc/pam.d/LoginWrapper.sh with contents:

#!/bin/bash
if [[ $EUID -ne 0 ]]; then # Execute only when a user other than root logs in
/usr/bin/sudo /path/to/your/root_script.sh
fi

Note that if your sudo is in a different path, you should update it above.

Now in /etc/sudoers add the line:

ALL ALL=(root) NOPASSWD: /path/to/your/root_script.sh

Solved this by adding all users to a group called mygroup

sudo groupadd mygroup

sudo usermod -aG mygroup user1

sudo usermod -aG mygroup user2

Then modifying /etc/sudoers to allow this group to execute sudo pkill without a password by adding the following to sudoers

%mygroup ALL=(ALL) NOPASSWD: /usr/bin/pkill

Then I just added the command sudo pkill to the bottom of /etc/profile and it worked a charm.