To be fair, the latest supybot release (0.83.4.1) is from 2005 …
Since that release everything (as little as that is) happens in a git repository at SourceForge.
A list of what's wrong:
Anyone can crash Supybot:
Just do something like this:
!misc last --regexp m/(.*\w){500}/
You can't unload misc without editing the config.
or
!math calc factorial(9999999)
(you may unload that one though).
I heard these are also supposed to bring the host the bot is running on down. I haven't noticed something like that though.
Anyone can access network services via Supybot
Through nesting format cut and misc tell this should be possible (I haven't found a way yet).
Thus, if your supybot has Operator privileges anyone can abuse these.
Furthermore anyone can change the password of the user the bot is on.
Alternative
There is a fork of Supybot called Limnoria where these issues are fixed.
You don't have to worry about compatibility, as a supybot config is compatible with a Limnoria config.