Not if the browser has been specifically given the cert. It is typical on corporate networks send all HTTP traffic to a router that acts on behalf of the outsi

Not if the browser has been specifically given the cert. It is typical on corporate networks send all HTTP traffic to a router that acts on behalf of the outside world. If your browser (and browsers these days relying on OS installed certs) has a cert validating the router, it will be legitimate: you have a valid cert for the router you're talking to. The trick is to use a piece of software that doesn't use your system's certs but has it's own built in, like run Linux in a VM and use that browser. That way, you'll see the MitM attack.