MSC3939: Account locking (#3939)

* MSCXXXX: Account locking

* MSC number

* Switch to 401 + soft_lougout

* FCP updates/clarity

* Mention appeals/information

* v1.10

* Clarify why not 403

* Update proposals/3939-account-locking.md

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

* Clarify UI expectations

* Update proposals/3939-account-locking.md

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Update proposals/3939-account-locking.md

Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

* Update proposals/3939-account-locking.md

Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

---------

Co-authored-by: Mathieu Velten <mathieuv@matrix.org>
Co-authored-by: Travis Ralston <travisr@matrix.org>
Co-authored-by: David Baker <dbkr@users.noreply.github.com>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

proposals/3939-account-locking.md

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+# MSC3939: Account locking
+
+Account locking is a common safety and security tool where server administrators
+can prevent a user from usefully using their account. For example, too many failed
+login attempts, escalation in moderation action, or out of band verification
+needing to be completed.
+
+Currently, Matrix only supports deactivating an account. This is a destructive
+action which often leads to the account leaving all joined rooms, among other
+details. With account locking, the effect of making the user unable to access
+their account is achieved without destroying that same account - the account
+can always be unlocked.
+
+This proposal covers account locking, though leaves the specifics of use as an
+implementation detail. APIs for administration of locking are also not provided.
+
+## Proposal
+
+When an account is locked, clients receive a `401 Unauthorized` error response
+to most APIs with an `M_USER_LOCKED` error code and `soft_logout` set to `true`.
+Excluded APIs are described below. We enable `soft_logout` to encourage clients
+to make use of the [soft logout](https://spec.matrix.org/v1.9/client-server-api/#soft-logout)
+semantics: keep encryption state, but otherwise render the account unusable. 401
+is used to support legacy clients by giving the user a semantically meaningful
+experience: they may need to try logging in again, and when they do they may get
+a more useful error message about their account status, though their session data
+may be deleted by the client if it doesn't recognize the error code. Soft logout
+aims to prevent total destruction of this data, however.
+
+Upon receiving the `M_USER_LOCKED` error, clients SHOULD retain session information
+including encryption state and inform the user that their account has been locked.
+Details about *why* the user's account is locked are not formally described by
+this proposal, though future MSCs which cover informing users about actions taken
+against their account should have such details. Clients may wish to make use of
+[server contact discovery](https://spec.matrix.org/v1.10/client-server-api/#getwell-knownmatrixsupport)
+in the meantime.
+
+Clients SHOULD hide the normal UI from the user when informing them that their
+account is locked, preventing general use of the account.
+
+Clients SHOULD continue to `/sync` and make other API calls to more quickly detect
+when the lock has been lifted.  However, clients should rate-limit their requests. If unlocked, the APIs will either return a different
+error code or a normal 200 OK/successful response. For example, `/sync` will return
+to working as though nothing ever happened. If the error code changes to
+`M_UNKNOWN_TOKEN`, the client should delete local session data as it normally
+would when seeing the error code (and use soft logout as it normally would). This
+is typically expected if the server admin logged the user out or the user logged
+themselves out.
+
+Locked accounts are still permitted to access the following API endpoints:
+
+* [`POST /logout`](https://spec.matrix.org/v1.9/client-server-api/#post_matrixclientv3logout)
+* [`POST /logout/all`](https://spec.matrix.org/v1.9/client-server-api/#post_matrixclientv3logoutall)
+
+When a user's account is locked, servers SHOULD NOT invalidate an account's access tokens in case the account becomes
+unlocked: the user should be able to retain their sessions without having to log
+back in. However, if a client requests a logout (using the above endpoints), the
+associated access tokens should be invalidated as normal.
+
+## Potential issues
+
+This proposal does not communicate *why* a user's account is restricted. The human-readable `error`
+field may contain some information, though anything comprehensive may not be surfaced to the user.
+A future MSC is expected to build a system for both informing the user of the action taken against
+their account and allow the user to appeal that action.
+
+## Alternatives
+
+[MSC3823](https://github.com/matrix-org/matrix-spec-proposals/pull/3823) covers
+a similar concept, though is semantically different. See [matrix-org/glossary](https://github.com/matrix-org/glossary)
+for details.
+
+Another similar concept would be "shadow banning", though this only applies to
+moderation use cases.
+
+A 403 HTTP status code was considered instead of 401 with a `soft_logout`. A 403
+would indicate that the given action is denied, but otherwise keep the user logged
+in. This could wrongly indicate [suspension](https://github.com/matrix-org/matrix-spec-proposals/pull/3823),
+confusing the user. Instead, we provide a semantically similar experience where
+the user gets soft logged out on legacy clients, preserving encryption and related
+session data (assuming the client also supports soft logout). This can result in
+some loss of other session data however, like device-specific settings. Users may
+also be differently confused when they try to log back in and get cryptic error
+messages (indicating wrong username/password), however as mentioned above in the
+Potential Issues section, communicating actions taken against an account is a
+concern for a future MSC.
+
+## Unstable prefix
+
+Until this proposal is considered stable, implementations must use
+`ORG_MATRIX_MSC3939_USER_LOCKED` instead of `M_USER_LOCKED`.