# Invalidating devices during password modification

There are multiple use cases for why a user might want to modify their password:

* Adopting a password manager (to use a unique password or more secure password).
* Password rotation.
* Re-secure a compromised account.
* ... probably tons of others ...

These can be summarized into two groups:

1. "My account has been compromised and I need to re-secure it."
2. "I just want to change my password."

The [current Matrix specification](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-account-password)
does not provide a way to differentiate between these use cases. It gives no
guidance into what should happen to other sessions / devices when a password is
modified and leaves it up to the implementation.

It is reasonable for a client to want to specify this behavior to offer two
different workflows:

1. Modify a password and log all other devices out (for use when an account has
   been compromised).
2. Modify a password and do not touch any session data (for use in a
   non-malicious situations).

Alternately a client may default to whichever workflow is best for their users.

## Proposal

An optional field is added to the JSON body body of the [password reset endpoint](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-account-password)
called `logout_devices`. This is a boolean flag (defaulting to `true`) that
signals to whether other devices and sessions should be invalidated after
modifying the password.

## Potential issues

While Synapse defaults to the behavior of invalidating other devices and
sessions this may not be true of other implementations. Thus, a default of
`true` may not be backwards compatible. It might be more prudent to specify that
the behavior of not specifying the `logout_devices` flag is undefined.

## Alternatives

Provide a new endpoint in a future version that supports an additional field (as
above), using a new endpoint would avoid backwards compatibility issues.

## Security considerations

By defaulting to invalidating devices and sessions the security considerations
of this endpoint should remain intact. A client will need to be modified to
choose to keep other devices active.