Add an MSC proposal for how to handle sessions when modifying a password.

proposals/2457-password-modification-invalidating-devices.md

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+# 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.