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Separate Endpoints for Binding Threepids

Note: This MSC obsoletes MSC2229, which dealt with changing the rules of the bind flag on POST /account/3pid. That endpoint is being deprecated as part of this MSC, thus MSC2229 is no longer relevant.

On the Client Server API there is currently a single endpoint for binding a threepid (an email or a phone number): POST /account/3pid. Depending on whether the bind flag is true or false, the threepid will be bound to either a user's account on the homeserver, or both the homeserver and an identity server. Note that we use the term add when talking about adding a threepid to a homeserver, and bind when binding a threepid to an identity server. This terminology will be used throughout the rest of this proposal.

Typically, when using the /account/3pid endpoint, the identity server handles the verification -- either by sending an email to an email address, or a SMS message to a phone number. Once completed, the homeserver will check with the identity server that verification had indeed happened, and if so, the threepid would be either added to the homeserver, or added to the homeserver and bound to the identity server simultaneously.

Now, consider the fact that the identity server used in this process is provided by the user, using the endpoint's id_server parameter. If the user were to supply a malicious identity server that would immediately answer "yes" to any threepid validation, then the user could add any threepid to their account on the homeserver (which is likely not something homeserver admins want).

To be clear, this is not a long-standing security issue. It is not a problem in any released version of Synapse, as Synapse keeps a list of "trusted identity servers" that acts a whitelist for what identity servers a user can specify.

The concept of this whitelist is being removed in this MSC however, as part of lessening the reliance of homeservers on identity servers. This cannot be done while the homeserver is still trusting an identity server for validation of threepids. If the endpoints are split, the homeserver will handle the validation of threepids being added to user accounts, and identity servers will validate threepids being bound to themselves.

Proposal

To solve this problem, two new endpoints will be added to the Client Server API: POST /account/3pid/bind and POST /account/3pid/add. Binding to an identity server will require standard authentication, whereas adding a 3pid to a user account will require User-Interactive Authentication. The latter is to prevent someone from adding a 3pid (which can be used to reset passwords) to someone who's left their account open on a public computer, without needing their password to do so.

Both endpoints will be rate-limited. The request parameters of POST /account/3pid/bind are the same as POST /account/3pid, minus the bind flag, and the contents of three_pid_creds have been brought to the top level of the request body. The request parameters of POST /account/3pid/add will simply consist of a JSON body containing client_secret and sid.

The homeserver should prevent a threepid being added to a user's account if it's already part of another user's account. However, the homeserver should not check for existing threepids when binding to an identity server. Identity servers do not enforce this requirement and neither should the proxying homeserver.

An example of binding a threepid to an identity server with this new endpoint is as follows:

First the client must request the threepid be validated by its chosen identity server.

POST https://identity.server/_matrix/identity/v2/validate/email/requestToken

{
    "client_secret": "don'tT3ll",
    "email": "bob@example.com",
    "send_attempt": 1
}

The identity server must send an email to the specified address, including a link to a URL on the identity server which will accept the validation session ID, the given client_secret, and a randomly-generated token.

Once an email has been sent, the user clicks the link in the email, which notifies the identity server that the email has been verified.

Next, the client completes the bind by calling the new endpoint on the homeserver:

POST https://home.server/_matrix/client/r0/account/3pid/bind

{
    "id_server": "example.org",
    "id_access_token": "abc123_OpaqueString",
    "sid": "abc123987",
    "client_secret": "don'tT3ll"
}

The homeserver will then make a bind request to the specified identity server on behalf of the user. The homeserver will record if the bind was successful and notify the user. The homeserver will remember this bind and the identity server it occurred on so that it can perform an unbind later if the user requests it or their account is deactivated.

The threepid has now been bound on the user's requested identity server without causing that threepid to be used for password resets or any other homeserver-related functions.

For completeness, here is an example of adding a threepid to the homeserver only, using the /account/3pid/add endpoint:

The homeserver is validating the threepid in this instance, so the client must use the /requestToken endpoint of the homeserver:

POST https://home.server/_matrix/client/r0/account/3pid/email/requestToken

{
    "client_secret": "don'tT3ll",
    "email": "bob@example.com",
    "send_attempt": 1,
}

Here the homeserver must send an email to the specified address, including a link to a URL on the homeserver which will accept the validation session ID, the given client_secret, and a randomly-generated token.

Once an email has been sent, the user clicks the link in the email, which notifies the homeserver that the threepid has been verified.

The client then sends a request to the endpoint on the homeserver to add the threepid to a user's account.

POST https://home.server/_matrix/client/r0/account/3pid/add

{
    "sid": "abc123987",
    "client_secret": "don'tT3ll"
}

The homeserver checks the threepid validation session referred to by the given ID and client_secret was validated, and if so adds the threepid to the user's account.

To achieve the above flows, some changes need to be made to existing endpoints. The POST /account/3pid endpoint is deprecated as the two new endpoints replace its functionality. The bind parameter is to be removed, with the endpoint functioning as if bind was false. Allowing an endpoint to add a threepid to both the identity server and homeserver at the same time requires one to trust the other, which is the exact behaviour we're trying to eliminate. Doing this also helps backward compatibility, as explained in Backwards compatibility.

Either the homeserver itself or a service that the homeserver delegates to should be handling the sending of validation messages, not a user-provided server. Any mention of the homeserver being able to proxy to an identity server in the below endpoint descriptions:

As well as the text "It is imperative that the homeserver keep a list of trusted Identity Servers and only proxies to those that it trusts." is to be removed from all parts of the spec, as the homeserver should no longer need to trust any identity servers.

Tradeoffs

One may question why clients don't just contact an identity server directly to bind a threepid, bypassing the implications of binding through a homeserver. While this will work, binds should still occur through a homeserver such that the homeserver can keep track of which binds were made, which is important when a user wishes to deactivate their account (and remove all of their bindings made on different identity servers).

A verification could occur on an identity server, which could then tell the homeserver that a validation happened, but then there are security considerations about how to authenticate an identity server in that instance (and prevent people pretending to be identity servers and telling homeservers about hundreds of fake threepid additions to a user's account).

Backwards compatibility

A new flag will be added to /versions' unstable_features section, m.separate_add_and_bind. If this flag is present and set to true, then clients should use the new API endpoints to carry out threepid adds and binds. If this flag is not present or set to false, clients should use /account/3pid, being aware that they can only bind threepids to the homeserver, not the identity server.

Old matrix clients will continue to use the /account/3pid endpoint. This MSC removes the bind parameter and forces /account/3pid calls to act as if bind was set to false. Old clients will still be able to add 3pids to the homeserver, but not bind to the identity server. New homeservers must ignore any id_server information passed to this endpoint.

Security considerations

Reducing the homeserver's trust in identity servers should be a boost to security and improve decentralisation in the Matrix ecosystem to boot.

Some endpoints of the Client Server API allow a user to provide an id_server parameter. Caution should be taken for homeserver developers to stop using these user-provided identity servers for any sensitive tasks where possible, such as password reset or account registration, if it removes the concept of a trusted identity server list.

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