pfSense

Just a thought -- but it would potentially be helpful if password validation logic would check for leading or trailing spaces in entered passwords and warn the user when saving by displaying the standard pfSense warning message at the top of the page. Make it a warning and not a fail, so the user can proceed with the space(s) if desired. Unintended spaces in passwords have been the source of several mysterious problems <grin>.

Passwords which begin with exclamation mark (!) are broken see:

https://forum.netgate.com/post/1216202

The proper fix would be to base64 encode the password before passing it to the command line and so avoid all the escaping issues.
The command would then do the decode before passing it to the kernel module.
In theory PPP passwords could contain all sorts of mad characters as all bytes values including NUL are valid. Passing them directly as a command line argument will always be dangerous.
Having the connection not work is probably the least worst thing that could happen.