In certain cases setuid() and similar can fail; if an environment limits
the number of processes a user can have, setuid() might fail if the target
uid already is at the limit.

A user who can execute processes with the same userid (for example by
having write access to CGI scripts) could clone() often; in this case
a lighttpd restart would end up with lighttpd running as root, and the
CGI scripts would run as root too.

It could be possible that remote users could trigger many processes too, and
trigger a restart using a different bug in lighttpd.