A weak password for root is foolish, regardless of the controls on 'su'. Even if user 'root' can only login at a console in a restricted machine room, I would not allow user 'root' to have a weak password.
I'd suggest disabling 'su' altogether and using 'sudo' for everything. By disabling, I mean any of:
- Exploiting any system-specific means of restricting access to 'su' (such as the group 'wheel' trick for BSD, or the Linux equivalent). Note that there is no formal standard for this; POSIX does not mandate the presence of 'su', for example.
- Remove it (
rm -f /bin/su).
- Remove its execute permission bits (
chmod o-x /bin/su or
chmod go-x /bin/su).
- Remove its setuid permission bit (
chmod u-s /bin/su).
The residual problem with disabling 'su' by removing it or removing permission bits is that some system scripts may depend on
su being present. There isn't a particularly clean solution for that - but they are generally few and far between because 'su' prompts for a password and prompting isn't liked in scripted environments. The other time 'su' is used is when 'root' runs the command to become another user; this is supported by removing the setuid bit (user root can run it, but no-one else can do so usefully). You might reinforce that by removing public and possibly group execute permission too (
chmod u-s,go-rwx /bin/su).
If you are not using one of the system-specific means, be very careful; test before putting this into production.