MSC2078: Sending Third-Party Request Tokens via the Homeserver

proposals/2078-homeserver-password-resets.md

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+# MSC2078 - Sending Third-Party Request Tokens via the Homeserver
+
+This MSC proposes removing the current requirement of the identity server to
+send third-party request tokens, and allows homeservers to implement the
+functionality instead. These request tokens are used to verify the identity of
+the request author as an owner of the third-party ID (3PID). This can be used
+for binding a 3PID to an account, or for resetting passwords via email or SMS.
+The latter is what this proposal mainly focuses on, but be aware that it allows
+for any task that requires requesting a token for a 3PID to be taken on by the
+homeserver instead of the identity server.
+
+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 3PID. This is due to the identity server handling the job of confirming the
+user's control of that identity.
+
+The MSC seeks to clarify that homeservers can take on the responsibility of
+sending password reset tokens themselves, and a new response field that will
+aid homeservers in doing so.
+
+# Background
+
+Currently when a client requests a 3PID token, it makes a call to one of the
+`/requestToken` endpoints on the homeserver. For instance, during password
+resets, a token is requested from 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),
+depending on the medium of the 3PID. These requests are supplied all the
+necessary details as well as an `id_server` field containing the domain address
+of a identity server trusted by the homeserver.
+
+In order to facilitate these requests, the homeserver will simply proxy them to
+the identity server. The IS will send out a token via email or sms, the user
+will click a link or enter the token into their client, and either the client
+or the user's browser will make a request **directly to the identity server**
+with the token for verification. The IS then informs the homeserver that
+verification was successful. At this point you can likely see that there is
+potential for abuse here, so instead Homeservers should be given the option to
+stop proxying the request to the identity server, and instead just send and
+validate the token themselves.
+
+## Proposal
+
+The homeserver should be allowed to either proxy `/requestToken` requests or
+handle them itself. Specifically, this means that the homeserver can both send
+password reset tokens (via email or SMS), as well as accept requests on an
+arbitrary endpoint (with the same parameters as
+[/_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.
+
+One additional complication that in the case of SMS, just a code is sent to a
+person's phone. This is then given to the client, but the client may not know
+where to send the code now, as it doesn't know whether the homeserver or
+identity server generated it.
+
+In order to combat this problem, the field `submit_url` MUST be added in the
+response from all of the variants of `/requestToken` in the 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).
+This URL is simply where the client should submit this token. The endpoint
+should accept the same parameters as
+[/_matrix/identity/api/v1/validate/{3pid_type}/submitToken](https://matrix.org/docs/spec/identity_service/r0.1.0.html#post-matrix-identity-api-v1-validate-email-submittoken)
+in the Identity Service API. The only recommendation to homeserver developers
+for this endpoint's path is to not be exactly the same as that of the identity
+server, in order to prevent clashes between setups running both an identity
+server and homeserver on the same domain.  If `submit_url` is omitted, the
+client MUST 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 MUST
+accept a token from user input, then make a POST request to the content of
+`submit_url` with the `sid`, `client_secret` and user-entered token.
+`submit_url` can lead to anywhere the homeserver deems necessary for
+verification. To be clear the content of `id_server` does not matter here, the
+client should just submit a POST request to the value of `submit_url`. Additionally 
+data MUST be submitted as a JSON body.
+
+An example exchange from the client's perspective is shown below:
+
+```
+POST https://homeserver.tld/_matrix/client/r0/account/password/email/requestToken
+
+{
+  "client_secret": "monkeys_are_AWESOME",
+  "email": "[email protected]",
+  "send_attempt": 1,
+  "id_server": "id.example.com"
+}
+```
+
+If the server responds with a `submit_url` field, it means the client should
+collect a token from the user and then submit it to the provided URL.
+
+```
+{
+  "sid": "123abc",
+  "submit_url": "https://homeserver.tld/_homeserver/password_reset/msisdn/submitToken"
+}
+```
+
+Since a `submit_url` was provided, the client will now collect a token from the
+user, say "123456", and then submit that as a POST request to the
+`"submit_url"`.
+
+```
+POST https://homeserver.tld/_homeserver/password_reset/msisdn/submitToken
+
+{
+  "sid": "123abc",
+  "client_secret": "monkeys_are_AWESOME",
+  "token": "123456"
+}
+```
+
+The client will then receive an appropriate response:
+
+```
+{
+  "success": true
+}
+```
+
+If the client did not receive a `submit_url` field, they should instead assume
+that verification will be completed out of band (e.g. the user clicks a link in
+their email and makes the submitToken request with their web browser).
+
+## Dismissed Alternatives
+
+Consideration was taken not to make `id_server` an optional field. Let's
+assume for a moment that it was optional. Now, a client could send a request to
+`/requestToken` omitting the `id_server` field. The homeserver however has
+opted to continue proxying `/requestToken` to the identity server, even though
+it knows this is potentially insecure. The homeserver now has no idea which
+identity server to proxy the request to, and must return a failure to the
+client. The client could then make another request with an `id_server`, but
+we've now made two requests that ended up in the same outcome, instead of one,
+in hopes of saving a very small amount of bandwidth by omitting the field
+originally.
+
+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.
+
+## 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.
+