cron_popen is very very trusting, and has multiple problems:

1) for fun, on a system not accepting mail, set
MAILTO=" -bd" and put in an entry like * * * * * ls
wait a minute, and ps -ax | grep sendmail... Now try using
your imagination.

2) there's an overflowable buffer where if you put more than
100 space-seperated items into MAILTO, you get a nice segv.
Looks quite exploitable.