Only the very largest companies in the world use a concept of a looking glass. A looking glass is a script that is designed to be a helpful tool to network administrators. Let’s say I’m super-massive-ISP A that wants to talk to super-massive-ISP B and we are peers. If the connection goes down, it’s often unclear as to whom is at fault for the network outage.
So each super-massive-ISP sets up a script called a “looking glass” that is designed to run commands on border routers. The commands are simple things that attempt to diagnose network connectivity issues. The problem is that these script are open source, and often poorly written. It’s not inconceivable that these looking glass scripts have a vulnerability that allows you to run arbitrary commands on the router.