I believe this should be exploitable in a typical MiTM scenario, espespecially if you don't use HSTS headers (also no need to downgrade as itsit's in HTTP anyways) -- a typical DNS spoofing attack can make a victim's IP of the same local network as localhost:8888 navigation leading to believe itsit's you and responding with the CORS response. -- I really don't see why you need a site with this kind of CORS response header