There exist several "family" apps available for mobile devices that allow family things to be shared: shopping lists, calendars, occasionally device locations, that kind of thing. At least two of these apps (and if it's at least two, I'm worried it's all of them) use the highly interesting practice of requiring users part of the same "family" group to use the same password (different usernames, though).

The justification given by at least one of these apps is that the password is used to determine which group the user belongs to, but I'm a programmer and I'm pretty sure that explanation passed through the backside of a male cow. What I am not, however, is a security expert, and while I'm pretty sure that enforcing shared passwords is a bad idea, I can't put into words why it's a bad idea.


It is a bad idea all around. First and foremost, it goes against the principle of authentication. By definition, the goal of authentication is to verify that a user is indeed a user, not his family member or a support technician. If someone else needs access, the should have access from their own account with their own password. At the very least, logging actions by a user would be meaningless. If the file says last edited by Bob, it could still have been anyone, no accountability.

Another problem with sharing a password is a technical one. Passwords should be salted and hashed before being stored in general, so there should be no way to find users with the same password. If they group users by it, they are storing passwords wrong.

And what happens if lets say Bob thinks someone may have seen him type the password or he may have saved it on a public computer? Does he frantically try to contact everyone to change password? Or what if he likes to change it preemptively?

Of course there could be an exception. That is if the password was not for authentication but for example for encryption. Lets say you have a cloud application that allows to share files with family. The files could be encrypted with the password end to end allowing the company to newer know the password in the first place. This would be arguably more secure, as it would be end-to-end encryption with nice strong symmetric cipher. On the other hand, it is still often possible to encrypt multiple copies of a master key to allow different passwords.


What if my family uses a password that happens to be the same as the one used by some other (unknown and unrelated) family? Are we all one big family now, and share one big shopping list?

(Of course all the security problems that Peter Harmann mentioned are also all true, but this one should be somewhat easier to explain even to a non-security person.)

  • Those families would still be distinguished by username. – voodoo-burger Jun 21 '19 at 22:28
  • That is not at all apparent from the question. Specifically, it says "that the password is used to determine which group the user belongs to", which directly means that if I happen to accidentally use the same password as you did, you and I would be considered part of the same group. – user88917 Jul 18 '19 at 3:14

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