Comment by EGreg

Right. But it can’t, for several reasons: 1) The proxy is open source and much simpler than PHP runtime or a web application. Its code only takes input and generates output. It is basically a pipe. It has no other capabilities to exploit. 2) The operating system, such as BSD or Linux, supports running processes under a user and enforces limits on that user. So you just jail this proxy — since it has one purpose. Unlike your application, which may do many things and you can’t restrict it, it may become a confused deputy