136 lines
7.3 KiB
Markdown
136 lines
7.3 KiB
Markdown
# MSC2778: Providing authentication method for appservice users
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Appservices within Matrix are increasingly attempting to support End-to-End Encryption. As such, they
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need a way to generate devices for their users so that they can participate in E2E rooms. In order to
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do so, this proposal suggests implementing an appservice extension to the
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[`POST /login` endpoint](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-login).
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Appservice users do not usually need to log in as they do not need their own access token, and do not
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traditionally need a "device". However, E2E encryption demands that at least one user in a room has a
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Matrix device which means bridge users need to be able to generate a device on demand. In the past,
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bridge developers have used the bridge bot's device for all bridge users in the room, but this causes
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problems should the bridge wish to only join ghosts to a room (e.g. for DMs).
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Another advantage this provides is that an appservice can now be used to generate access tokens for
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any user in its namespace without having to set a password for that user, which may be useful where
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maintaining password(s) in the configuration is undesirable.
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## Proposal
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A new `type` is to be added to `POST /login`: `m.login.application_service`
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The `/login` endpoint may now take an `access_token` in the same way that other
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authenticated endpoints do. No additional parameters should be specified in the request body.
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Example request
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```json
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{
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"type": "m.login.application_service",
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"identifier": {
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"type": "m.id.user",
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"user": "_bridge_alice"
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}
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}
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```
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Note: Implementations MUST use the `identifier.type`=`m.id.user` method of specifying the
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localpart. The deprecated top-level `user` field **cannot** use this login flow type. This
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is deliberate so as to coax developers into using the new identifier format when implementing
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new flows.
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The response body should be unchanged from the existing `/login` specification.
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If one of the following conditions are true:
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- The access token is not provided
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- The access token does not correspond to an appservice
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- Or the user has not previously been registered
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Then the servers MUST reject with HTTP 403, with an `errcode` of `"M_FORBIDDEN"`.
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If the access token DOES correspond to an appservice but the user is not inside its namespace,
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then the `errcode` must be `"M_EXCLUSIVE"`.
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Homeservers should ignore the `access_token` parameter if a type other than
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`m.login.application_service` has been provided.
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Appservices creating **new** users can still use the `/register` endpoint to generate an `access_token` / `device_id`
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but for existing users, the `/login` endpoint can be used instead.
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## Potential issues
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This proposal means that there will be more calls to make when setting up a appservice user, when
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using encryption. While this could be done during the registration step, this would prohibit creating
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new devices should the appservice intentionally or inadvertently have lost the client-side device data.
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## Alternatives
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### 1. Include the token in the `/login` request body
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One minor tweak to the current proposal could be to include the token as part of the auth data, rather than
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being part of the header/params to the request. An argument could be made for either, but since the specification
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expects the appservice to pass the token this way in all requests, including `/register`, it seems wise to keep
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it that way.
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### 2. Use implementation specific "shared secret" authentication
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Some community members have used homeserver implementation details such as a "shared secret" authentication method to
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log into the accounts without having to use the /login process at all. Synapse provides such a function,
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but also means the appservice can now authenticate as any user on the homeserver. This is undesirable from a
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security standpoint.
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### 3. Keep using `/register` solely
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A third option could be to create a new endpoint that simply creates a new device for an appservice user on demand.
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Given the rest of the matrix eco-system does this with /login, and /login is already extensible with `type`, it would
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create more work for all parties involved for little benefit.
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Finally, `POST /register` does already return a `device_id` and `access_token` so appservices
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could store this information rather than calling `POST /login` at all. This does however present a few problems:
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- Quite a few appservices which only support unencrypted messaging do not use/store the `device_id`/`access_token` from a register call.
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In the event that an appservice eventually gains the ability to support encryption, they would be unable to fetch a new `device_id`/
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`access_token` for any existing users (as `/register` would fail for an existing user).
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- If user tokens were lost or exposed, there is no way to programmatically create new access tokens for these users.
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- Finally, if a user was registered externally and the appservice would like to masquerade as it, it would be unable to fetch
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an access token for that user.
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While `POST /register` does work, it is impactical as the sole method of fetching an access token.
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## Security considerations
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Appservices could use this new functionality to generate devices for any userId that are within its namespace e.g. setting the
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user namespace regex to `@.*:example.com` would allow appservice to control anyone on the homeserver. While this sounds scary, in practice
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this is not a problem because:
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- Appservice namespaces are maintained by the homeserver admin. If the namespace were to change, then it's reasonable
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to assume that the server admin is aware. There is no defense mechanism to stop a malicious server admin from creating new
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devices for a given user's account as they could also do so by simply modifying the database.
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- While an appservice *could* try to masquerade as a user maliciously without the server admin expecting it, it would still
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be bound by the restrictions of the namespace. Server admins are expected to be aware of the implications of adding new
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appservices to their server so the burden of responsibility lies with the server admin.
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- Appservices already can /sync as any user using the `as_token` and send any messages as any user in the namespace, the only
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difference is that without a dedicated access token they are unable to receive device messages. While in theory this
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does make them unable to see encrypted messages, this is not designed to be a security mechanism.
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In conclusion this MSC only automates the creation of new devices for users inside an AS namespace, which is something
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a server admin could already do. Appservices should always be treated with care and so with these facts in mind the MSC should
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be considered secure.
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## Unstable prefix
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Implementations should use `uk.half-shot.msc2778.login.application_service` for `type` given in the
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`POST /login` until this lands in a released version of the specification.
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## Implementations
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The proposal has been implemented by a homeserver, a bridge SDK and two bridges:
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- [synapse](https://github.com/matrix-org/synapse/pull/8320)
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- [mautrix-python](https://github.com/tulir/mautrix-python/commit/12d7c48ca7c15fd3ff61608369af1cf69e289aeb)
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- [mautrix-whatsapp](https://github.com/tulir/mautrix-whatsapp/commit/ead8a869c84d07fadc7cfcf3d522452c99faaa36)
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- [matrix-appservice-bridge](https://github.com/matrix-org/matrix-appservice-bridge/pull/231/files#diff-5e93f1b51d50a44fcf0ca46ea1793c1cR851-R864)
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