MSC3916: Authentication for media

proposals/3916-authentication-for-media.md

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+# MSC3916: Authentication for media access, and new endpoint names
+
+Currently, access to media in Matrix has a number of problems including the following:
+
+* The only protection for media is the obscurity of the URL, and URLs are
+  easily leaked (eg accidental sharing, access
+  logs). [synapse#2150](https://github.com/matrix-org/synapse/issues/2150)
+* Anybody (including non-matrix users) can cause a homeserver to copy media
+  into its local
+  store. [synapse#2133](https://github.com/matrix-org/synapse/issues/2133)
+* When a media event is redacted, the media it used remains visible to all.
+  [synapse#1263](https://github.com/matrix-org/synapse/issues/1263)
+* There is currently no way to delete
+  media. [matrix-spec#226](https://github.com/matrix-org/matrix-spec/issues/226)
+* If a user requests GDPR erasure, their media remains visible to all.
+* When all users leave a room, their media is not deleted from the server.
+
+These problems are all difficult to address currently, because access to media
+is entirely unauthenticated. The first step for a solution is to require user
+authentication. Once that is done, it will be possible to impose authorization
+requirements to address the problems mentioned above. (See, for example,
+[MSC3911](https://github.com/matrix-org/matrix-spec-proposals/pull/3911) which
+builds on top of this MSC.)
+
+This proposal supersedes [MSC1902](https://github.com/matrix-org/matrix-spec-proposals/pull/1902).
+
+## Proposal
+
+1. New endpoints
+
+   The existing `/_matrix/media/v3/` endpoints are to be deprecated, and replaced
+   by new endpoints under the `/_matrix/client` and `/_matrix/federation`
+   hierarchies.
+
+   The table below shows a mapping between old and new endpoint:
+
+   | Old                                                                                                                                                              | Client-Server                                                             | Federation                                                          |
+   | ---------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------- | ------------------------------------------------------------------- |
+   | [`GET /_matrix/media/v3/preview_url`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3preview_url) | `GET /_matrix/client/v1/media/preview_url` | - |
+   | [`GET /_matrix/media/v3/config`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3config) | `GET /_matrix/client/v1/media/config` | - |
+   | [`GET /_matrix/media/v3/download/{serverName}/{mediaId}`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3downloadservernamemediaid)            | `GET /_matrix/client/v1/media/download/{serverName}/{mediaId}`            | `GET /_matrix/federation/v1/media/download/{serverName}/{mediaId}`  |
+   | [`GET /_matrix/media/v3/download/{serverName}/{mediaId}/{fileName}`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3downloadservernamemediaidfilename) | `GET /_matrix/client/v1/media/download/{serverName}/{mediaId}/{fileName}` | -                                                                 |
+   | [`GET /_matrix/media/v3/thumbnail/{serverName}/{mediaId}`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3thumbnailservernamemediaid)          | `GET /_matrix/client/v1/media/thumbnail/{serverName}/{mediaId}`           | `GET /_matrix/federation/v1/media/thumbnail/{serverName}/{mediaId}` |
+
+   **Note**: [`POST /_matrix/media/v3/upload`](https://spec.matrix.org/v1.6/client-server-api/#post_matrixmediav3upload)
+   is **not** modified by this MSC: it is intended that it be brought into line with the other
+   endpoints by a future MSC, such as [MSC3911](https://github.com/matrix-org/matrix-spec-proposals/pull/3911).
+
+2. Removal of `allow_remote` parameter from `/download`
+
+   The current
+   [`/download`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3downloadservernamemediaid)
+   and
+   [`/thumbnail`](https://spec.matrix.org/v1.6/client-server-api/#get_matrixmediav3thumbnailservernamemediaid)
+   endpoints take an `allow_remote` query parameter, indicating whether the
+   server should request remote media from other servers. This is redundant
+   with the new endpoints, so will not be supported.
+
+   Servers should never return remote media from `GET
+   /_matrix/federation/v1/media/download` or `GET
+   /_matrix/federation/v1/media/thumbnail`; indeed, the `serverName` is
+   included in the URI only for consistency with the CS-API.
+
+   `/_matrix/client/v1/media/download` and
+   `/_matrix/client/v1/media/thumbnail` should return remote media as normal.
+
+3. Authentication on all endpoints
+
+   Currently, the `/download` and `/thumbnail` endpoints have no authentication
+   requirements. Under this proposal, the new endpoints will be authenticated
+   the same way as other endpoints: they will require an `Authorization` header
+   which must be `Bearer {accessToken}` for `/_matrix/client`, or the signature
+   for `/_matrix/federation`.
+
+4. Updated response format
+
+   * For the new `/_matrix/client` endpoints, the response format is the same as
+     the corresponding original endpoints.
+
+   * To enable future expansion, for the new `/_matrix/federation` endpoints,
+     the response is
+     [`multipart/mixed`](https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html)
+     content with two parts: the first must be a JSON object (and should have a
+     `Content-type: application/json` header), and the second is the media item
+     as per the original endpoints.
+
+     No properties are yet specified for the JSON object to be returned.
+
+     An example response:
+
+     ```
+     Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08jU534c0p
+
+     --gc0p4Jq0M2Yt08jU534c0p
+     Content-Type: application/json
+
+     {}
+
+     --gc0p4Jq0M2Yt08jU534c0p
+     Content-Type: text/plain
+
+     This media is plain text. Maybe somebody used it as a paste bin.
+
+     --gc0p4Jq0M2Yt08jU534c0p
+     ```
+
+5. Backwards compatibility mechanisms
+
+   a. Backwards compatibility with older servers: if a client or requesting
+   server receives a 404 error with a non-JSON response, or a 400 or 404 error with
+   `{"errcode": "M_UNRECOGNIZED"}`, in response to a request to one of the new
+   endpoints, they may retry the request using the original endpoint.
+
+   b. Backwards compatibility with older clients and federating servers:
+   servers may for a short time choose to allow unauthenticated access via the
+   deprecated endpoints.
+
+### Effects on client applications
+
+Naturally, implementations will be required to provide `Authorization` headers
+when accessing the new endpoints. This will be simple in some cases, but rather
+more involved in others. This section considers some of those cases.
+
+#### IRC/XMPP bridges
+
+Possibly the largest impact will be on IRC and XMPP bridges.  Since IRC and
+XMPP have no media repository of their own, these bridges currently transform
+`mxc:` URIs into `https://<server>/_matrix/media/v3/download/` URIs and forward
+those links to the remote platform. This will no longer be a viable option.
+
+One potential solution is for the bridges to provide a proxy.
+
+In this scenario, the bridge would have a secret HMAC key. When it
+receives a matrix event referencing a piece of media, it should create a new URI
+referencing the media, include an HMAC to prevent tampering. For example:
+
+```
+https://<bridge_server>/media/{originServerName}/{mediaId}?mac={hmac}
+```
+
+When the bridge later receives a request to that URI, it checks the hmac,
+and proxies the request to the homeserver, using its AS access
+token in the `Authorization` header.
+
+The bridge might also choose to embed information such as the room that
+referenced the media, and the time that the link was generated, in the URL.
+Such mechanisms would allow the bridge to impose controls such as:
+
+* Limiting the time a media link is valid for. Doing so would help prevent
+  visibility to users who weren't participating in the chat.
+
+* Rate-limiting the amount of media being shared in a particular room (in other
+  words, avoiding the use of Matrix as a Warez distribution system).
+
+#### Icons for "social login" flows
+
+When a server supports multiple login providers, it provides the client with
+icons for the login providers as `mxc:` media URIs. These must be accessible
+without authentication (because the client has no access token at the time the
+icons are displayed).
+
+This remains a somewhat unsolved problem. Possibly the clients can continue
+to call the legacy `/_matrix/media/v3/download` URI for now: ultimately this
+problem will be solved by the transition to OIDC. Alternatively, we may need
+to provide an alternative `/_matrix/client/v3/login/sso/icon/{idpId}` API
+specifically for access to these icon.
+
+(This was previously discussed in
+[MSC2858](https://github.com/matrix-org/matrix-spec-proposals/pull/2858#discussion_r543513811).)
+
+## Potential issues
+
+* Setting the `Authorization` header is going to be annoying for web clients. Service workers
+  might be needed.
+
+* Users will be unable to copy links to media from web clients to share out of
+  band. This is considered a feature, not a bug.
+
+## Alternatives
+
+* Allow clients to upload media which does not require authentication (for
+  example via a `public=true` query parameter). This might be particularly
+  useful for IRC/XMPP bridges, which could upload any media they encounter to
+  the homeserver's repository.
+
+  The danger with this is that is that there's little stopping clients
+  continuing to upload media as "public", negating all of the benefits in this
+  MSC. It might be ok if media upload it was restricted to certain privileged
+  users.
+
+* We could simply require that `Authorization` headers be given when calling
+  the existing endpoints. However, doing so would make it harder to evaluate
+  the proportion of clients which have been updated, and it is a good
+  opportunity to bring these endpoints into line with the rest of the
+  client-server and federation APIs.
+
+* There's no real need to rename `GET /_matrix/media/v3/preview_url` and `GET
+  /_matrix/media/v3/config` at present, and we could just leave them in
+  place. However, changing them at the same time makes the API more consistent.
+
+  Conversely, we should make sure to rename `POST
+  /_matrix/media/v3/upload`. The reason to delay doing so is because MSC3911
+  will make more substantial changes to this endpoint, requiring another
+  rename, and it is expected that both proposals will be mergeed at the same
+  time (so a double rename will be confusing and unnecessary). However, if
+  MSC3911 is delayed or rejected, we should reconsider this.
+
+* Rather than messing with multipart content, have a separate endpoint for
+  servers to get the metadata for a media item. That would mean two requests,
+  but might make more sense than both `/thumbnail` and `/download` providing
+  the info.
+
+## Unstable prefix
+
+While this proposal is in development, the new endpoints should be named as follows:
+
+* `GET /_matrix/client/unstable/org.matrix.msc3916/media/preview_url`
+* `GET /_matrix/client/unstable/org.matrix.msc3916/media/config`
+* `GET /_matrix/client/unstable/org.matrix.msc3916/media/download/{serverName}/{mediaId}`
+* `GET /_matrix/client/unstable/org.matrix.msc3916/media/download/{serverName}/{mediaId}/{fileName}`
+* `GET /_matrix/client/unstable/org.matrix.msc3916/media/thumbnail/{serverName}/{mediaId}`
+* `GET /_matrix/federation/unstable/org.matrix.msc3916/media/download/{serverName}/{mediaId}`
+* `GET /_matrix/federation/unstable/org.matrix.msc3916/media/thumbnail/{serverName}/{mediaId}`
+
+## Dependencies
+
+None at present.