Question

I’d read on a few Tomcat guides to run Tomcat as a user with nologin as the shell in order to prevent this user from being able to use the shell if there was a security breach in Tomcat.

I’d tried executing a script as this user with the setuid (-rws---r-x) and also by using su -s /bin/sh my_nologin_user myscript.sh but I am finding that it is able to execute commands like pwd
ps -ef
grep .

Clearly I am misunderstanding this but I can’t see what the point of using nologin as the shell is if the Tomcat startup script could be edited to perform any shell command (if hacked).

Answer 1

The nologin shell prevents just that: login. Any system login service — various ttys, sshd, etc, will deny access because they run the login executable, and that fails. A nologin shell won’t prevent one from running arbitrary commands through a shell once untrusted code is running under that user id.

The point of using nologin as the shell is that someone can’t ssh/telnet into your system as the user in question — as long as said services use the default configuration and run login on the (possibly pseudo) tty.