Implementing security in crontab is something easy to do because all the tools are already available with the operating system, this info is usefull in Solaris as in Linux, and probably many other *nix like systems.

To give or deny access to crontab, this means, give the user permission to modify its own crontab file (see Scheduling processes in Solaris) two files have to be modified:
/etc/cron.d/cron.deny
/etc/cron.d/cron.allow
This files contain one username per line, the cron.allow file contains the usernames that have access to cron, and, if this file exists, the cron.deny is not checked.

The behavior is this: