¿Es una amenaza grave? ¿Qué hacer para evitarla?
La siguiente cita de seguridad de Google explica este problema:
Unfortunately, we believe that this class of attacks is inherent to the current design of web browsers and can't be meaningfully mitigated by any single website; in particular, clobbering the window. opener property limits one of the vectors but still makes it easy to exploit the remaining ones.