It seems that entry to House common rooms is through audibly validating a password with whoever guards the door.
Is this the case for all Houses?
If so, would it be easy for someone to conceal themselves around the entrance to the common room and gain entry to said room by overhearing the password, even if only until the password is again changed?
I guess you might ask why would a student want to enter another House's common room (maybe to impersonate someone via Polyjuice Potion and obtain secrets) in their day-to-day lives at Hogwarts, which is true. But, how is it not more rampant? Are there other protections in place?