Mastering Linux Security and Hardening

Preventing users from using shell escapes

Certain programs, especially text editors and pagers, have a handy shell escape feature. This allows a user to run a shell command without having to exit the program first. For example, from the command mode of the Vi and Vim editors, someone could run the ls command by doing :!ls. Executing the command would look like this:

# useradd defaults fileGROUP=100HOME=/homeINACTIVE=-1EXPIRE=SHELL=/bin/bashSKEL=/etc/skelCREATE_MAIL_SPOOL=yes~~:!ls

The output would look like this:

[donnie@localhost default]$ sudo vim useradd[sudo] password for donnie:grub nss useraddPress ENTER or type command to continuegrub nss useraddPress ENTER or type command to continue

Now, imagine that you want Frank to be able to ...