Sorry. I'm just doing a bad job writing my questions of late.

I'm not trying to prevent the attack where a baddie steals someone's credentials. I'm trying to prevent an attack where they inject something I didn't foresee and crack my privileged process. Just think "SQL injection." If someone says their username is "bob; rm -r *", I don't want to do something dumb like system('chdir /home/'.$username). (This is a dumb example. I wouldn't ever let user-input that close to a system() call.)

I figure the first step in preventing this kind of attack is narrowing the protocol of communications between the client and server.

--Pileofrogs