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MSC2778: Providing authentication method for appservice users

Appservices within Matrix are increasingly attempting to support End-to-End Encryption. As such, they need a way to generate devices for their users so that they can participate in E2E rooms. In order to do so, this proposal suggests implementing an appservice extension to the POST /login endpoint.

Appservice users do not usually need to log in as they do not need their own access token, and do not traditionally need a "device". However, E2E encryption demands that at least one user in a room has a Matrix device which means bridge users need to be able to generate a device on demand. In the past, bridge developers have used the bridge bot's device for all bridge users in the room, but this causes problems should the bridge wish to only join ghosts to a room (e.g. for DMs).

Another advantage this provides is that an appservice can now be used to generate access tokens for any user in its namespace without having to set a password for that user, which may be useful where maintaining password(s) in the configuration is undesirable.

Proposal

A new type is to be added to POST /login: m.login.application_service

The /login endpoint may now take an access_token in the same way that other authenticated endpoints do. No additional parameters should be specified in the request body.

Example request

{
  "type": "m.login.application_service",
  "identifier": {
    "type": "m.id.user",
    "user": "_bridge_alice"
  }
}

Note: Implementations MUST use the identifier.type=m.id.user method of specifying the localpart. The deprecated top-level user field cannot use this login flow type. This is deliberate so as to coax developers into using the new identifier format when implementing new flows.

The response body should be unchanged from the existing /login specification.

If one of the following conditions are true:

The access token is not provided
The access token does not correspond to an appservice
Or the user has not previously been registered

Then the servers MUST reject with HTTP 403, with an errcode of "M_FORBIDDEN".

If the access token DOES correspond to an appservice but the user is not inside its namespace, then the errcode must be "M_EXCLUSIVE".

Homeservers should ignore the access_token parameter if a type other than m.login.application_service has been provided.

Appservices creating new users can still use the /register endpoint to generate an access_token / device_id but for existing users, the /login endpoint can be used instead.

Potential issues

This proposal means that there will be more calls to make when setting up a appservice user, when using encryption. While this could be done during the registration step, this would prohibit creating new devices should the appservice intentionally or inadvertently have lost the client-side device data.

Alternatives

1. Include the token in the `/login` request body

One minor tweak to the current proposal could be to include the token as part of the auth data, rather than being part of the header/params to the request. An argument could be made for either, but since the specification expects the appservice to pass the token this way in all requests, including /register, it seems wise to keep it that way.

2. Use implementation specific "shared secret" authentication

Some community members have used homeserver implementation details such as a "shared secret" authentication method to log into the accounts without having to use the /login process at all. Synapse provides such a function, but also means the appservice can now authenticate as any user on the homeserver. This is undesirable from a security standpoint.

3. Keep using `/register` solely

A third option could be to create a new endpoint that simply creates a new device for an appservice user on demand. Given the rest of the matrix eco-system does this with /login, and /login is already extensible with type, it would create more work for all parties involved for little benefit.

Finally, POST /register does already return a device_id and access_token so appservices could store this information rather than calling POST /login at all. This does however present a few problems:

Quite a few appservices which only support unencrypted messaging do not use/store the device_id/access_token from a register call. In the event that an appservice eventually gains the ability to support encryption, they would be unable to fetch a new device_id/ access_token for any existing users (as /register would fail for an existing user).
If user tokens were lost or exposed, there is no way to programmatically create new access tokens for these users.
Finally, if a user was registered externally and the appservice would like to masquerade as it, it would be unable to fetch an access token for that user.

While POST /register does work, it is impactical as the sole method of fetching an access token.

Security considerations

Appservices could use this new functionality to generate devices for any userId that are within its namespace e.g. setting the user namespace regex to @.*:example.com would allow appservice to control anyone on the homeserver. While this sounds scary, in practice this is not a problem because:

Appservice namespaces are maintained by the homeserver admin. If the namespace were to change, then it's reasonable to assume that the server admin is aware. There is no defense mechanism to stop a malicious server admin from creating new devices for a given user's account as they could also do so by simply modifying the database.
While an appservice could try to masquerade as a user maliciously without the server admin expecting it, it would still be bound by the restrictions of the namespace. Server admins are expected to be aware of the implications of adding new appservices to their server so the burden of responsibility lies with the server admin.
Appservices already can /sync as any user using the as_token and send any messages as any user in the namespace, the only difference is that without a dedicated access token they are unable to receive device messages. While in theory this does make them unable to see encrypted messages, this is not designed to be a security mechanism.

In conclusion this MSC only automates the creation of new devices for users inside an AS namespace, which is something a server admin could already do. Appservices should always be treated with care and so with these facts in mind the MSC should be considered secure.

Unstable prefix

Implementations should use uk.half-shot.msc2778.login.application_service for type given in the POST /login until this lands in a released version of the specification.

Implementations

The proposal has been implemented by a homeserver, a bridge SDK and two bridges:

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