MSC2078: Sending Third-Party Request Tokens via the Homeserver

proposals/2078-homeserver-password-resets.md

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+# MSC2078 - Sending Password Reset Emails via the Homeserver
+
+This MSC proposes removing the current requirement of the identity server to
+send password reset tokens, and allows homeservers to implement the
+functionality instead. The intention is to put less trust in the identity
+server which is currently one of the most centralised components of Matrix. As
+it stands, an attacker in control of a identity server can reset a user's
+password if the identity server is considered trusted by that homeserver, and
+the user has registered at least one third-party identifier (3PID). This is due
+to the identity server currently handling the job of confirming the user's
+control of that identity.
+
+The MSC aims to simply clarify that homeservers can take on the responisibility
+of sending password reset tokens themselves.
+
+## Proposal
+
+Currently when a client requests a password reset, they make a call to either
+[/_matrix/client/r0/account/password/email/requestToken](https://matrix.org/docs/spec/client_server/r0.4.0.html#post-matrix-client-r0-account-password-email-requesttoken)
+or
+[/_matrix/client/r0/account/password/msisdn/requestToken](https://matrix.org/docs/spec/client_server/r0.4.0.html#post-matrix-client-r0-account-password-msisdn-requesttoken).
+This request is supplied all the necessary details as well as a `id_server`
+field containing the address of a trusted identity server which the user has
+used in the past to bind their 3PID.
+
+The `id_server` field is currently required as the homeserver must know where
+to proxy the request to. This MSC proposes not to change the requirements of
+this field. Instead, it asks to clarify that the homeserver is allowed to not
+proxy the request, but carry it out itself. This would mean the homeserver can
+both send password reset tokens (via email or SMS), as well as accept requests
+to
+[/_matrix/identity/api/v1/validate/email/submitToken](https://matrix.org/docs/spec/identity_service/r0.1.0.html#post-matrix-identity-api-v1-validate-email-submittoken)
+to verify that token.
+
+An additional complication is that in the case of SMS, a full link to reset passwords is not sent, but a short code. The client then asks the user to enter this code, however the client may now not know where to send the code. Should it send it to the identity server or the homeserver? Which sent out the code?
+
+In order to combat this problem, the field `submit_url` should be added in the response from both the email and msisdn variants of the `/requestToken` Client-Server API, if and only if the verification message contains a code the user is expected to enter into the client (for instance in the case of a short code through SMS). If this field is omitted, the client should continue the same behaviour from before, which is to send the token to the identity server directly. This is intended for backwards compatibility with older servers.
+
+If the client receives a response to `/requestToken` with `submit_url`, it should accept the token from user input, then make a request (either POST or GET, depending on whether it desires a machine- or human-readable response) to the content of `submit_url` with the `sid`, `client_secret` and user-entered token. This data should be submitted as query parameters for `GET` request, and a JSON body for a `POST`.
+
+## Tradeoffs
+
+If homeservers choose to not proxy the request, they will need to implement the
+ability to send emails and/or SMS messages. This is left as a detail for the
+homeserver implementation.
+
+## Future Considerations
+
+At some point we should look into removing the `id_server` field altogether and
+removing any email/SMS message sending from the identity server. This would
+drastically reduce the amount of trust needed in the identity server and its
+required ability. This is, however, a good first step.