If you have superuser access on the box on which you are running Instiki, you can have a more secure setup by running Instiki under a new, unprivileged UID.
instiki, with no shell access.% sudo chown instiki public secret db db/production.db.sqlite3
% sudo chown -R instiki log storage cache
% sudo -u instiki ./instiki --daemon
% sudo -u instiki kill pid-of-Instiki
Instiki provides a mechanism for uploading files to your Wiki. This means, in principle, that miscreants could use your Instiki Wiki as a dropbox for sharing files on the internet.
To mitigate the threat, there is a default limit, of 100 KB, on the size of uploaded files. You can change this limit or — better, yet — disable file uploads on publicly accessible Webs in the Edit Web configuration page.