There is a way to very easily crash server using CC computer. Just make a new program with that code and run it:
Code removed, PM me if you want to test.
It will basically cycle through redstone states very fast and in few seconds server will crash with RedPower error. And if bad guy added that to the startup script and there is a chunkloader nearby, server will also crash almost immediately after starting up, rendering it useless.
Add this to the end of the /mods/ComputerCraft/lua/rom/startup :
oldRedstoneFunction = rs.setOutput
rs.setOutput = function(side, bool)
This will add small delay to the rs.setOutput function, which is just enough to solve this issue. I also recommend replacing oldRedstoneFunction with something else (just any word will do it), because if attacker knows this word, he can circumvent protection.
Explanation on how this fix works: CLICK