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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
Anybody (including non-matrix users) can cause a homeserver to copy media into its local store. synapse#2133
When a media event is redacted, the media it used remains visible to all. synapse#1263
There is currently no way to delete media. matrix-spec#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 which builds on top of this MSC.)

This proposal supersedes MSC1902.

Proposal

New endpoints

The existing /_matrix/media/v3/ endpoints become deprecated, and new endpoints under the /_matrix/client and /_matrix/federation hierarchies are introduced. Removal of the deprecated endpoints would be a later MSC under the deprecation policy.

The following table below shows a mapping between deprecated and new endpoint:

Deprecated	Client-Server	Federation
`GET /_matrix/media/v3/preview_url`	`GET /_matrix/client/v1/media/preview_url`	-
`GET /_matrix/media/v3/config`	`GET /_matrix/client/v1/media/config`	-
`GET /_matrix/media/v3/download/{serverName}/{mediaId}`	`GET /_matrix/client/v1/media/download/{serverName}/{mediaId}`	`GET /_matrix/federation/v1/media/download/{mediaId}`
`GET /_matrix/media/v3/download/{serverName}/{mediaId}/{fileName}`	`GET /_matrix/client/v1/media/download/{serverName}/{mediaId}/{fileName}`	-
`GET /_matrix/media/v3/thumbnail/{serverName}/{mediaId}`	`GET /_matrix/client/v1/media/thumbnail/{serverName}/{mediaId}`	`GET /_matrix/federation/v1/media/thumbnail/{mediaId}`

Note: POST /_matrix/media/v3/upload and POST /_matrix/media/v1/create are not modified or deprecated by this MSC: it is intended that they be brought into line with the other endpoints by a future MSC, such as MSC3911.

Removal of allow_remote parameter from /download and /thumbnail

The current /download and /thumbnail 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 MUST NOT return remote media from GET /_matrix/federation/v1/media/download or GET /_matrix/federation/v1/media/thumbnail. The serverName is omitted from the endpoint's path to strongly enforce this - the mediaId in a request is assumed to be scoped to the target server.

/_matrix/client/v1/media/download and /_matrix/client/v1/media/thumbnail return remote media as normal.
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.

Clients SHOULD NOT use the deprecated ?access_token query string authentication mechanism. The method is pending removal and is generally unsafe. See those MSCs for further details.

Note: This fixes matrix-spec#313.
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 content with exactly two parts: the first MUST be a JSON object (and should have a Content-type: application/json header), and the second MUST be the media item. The media item may be served inline, as shown in the first example below, or be a pointer to a URL containing the media item's bytes instead, represented by the Location header described further below.
  
  No properties are yet specified for the JSON object to be returned. One possible use is described by MSC3911.
  
  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
```
  The second part (media item bytes) MAY include a Location header to point to the raw media object instead of having bytes itself. Servers SHOULD NOT cache the Location header's value as the responding server may have applied time limits on its validity. Servers which don't immediately download the media from the provided URL should re-request the media and metadata from the /download endpoint when ready for the media bytes.
  
  The Location header's URL does not require authentication, as it will typically be served by a CDN or other non-matrix server (thus being unable to verify any X-Matrix signatures, for example).
  
  Note that all other headers besides Location for the media item part are ignored when Location is present. The Content-Type, Content-Disposition, etc headers will be served from the Location's URL instead. Similarly, the body for the media item part is ignored and SHOULD be empty.
  
  An example response with a Location redirect would be:
```
Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08jU534c0p

--gc0p4Jq0M2Yt08jU534c0p
Content-Type: application/json

{}

--gc0p4Jq0M2Yt08jU534c0p
Location: https://cdn.example.org/ab/c1/2345.txt

--gc0p4Jq0M2Yt08jU534c0p
```
  If the server were to curl https://cdn.example.org/ab/c1/2345.txt, it'd get something similar to the following:
```
Content-Type: text/plain

This media is plain text. Maybe somebody used it as a paste bin.
```
  Note: For clarity, the above applies to the federation /thumbnail endpoint as well as /download.
Backwards compatibility mechanisms

Servers SHOULD stop serving new media as unauthenticated within 1 spec release of this proposal being released itself using a standard 404 M_NOT_FOUND response. Existing media should continue to be served from the unauthenticated endpoints indefinitely for backwards compatibility. For example, if this proposal is released in Matrix 1.11, then by Matrix 1.12 servers should freeze the old unauthenticated endpoints by only serving media known to exist from before the freeze.

"New media" is any media which local users upload after the freeze is put in place, and any remote media which becomes cached after the freeze as well. This could be marked by a configuration option within the server software, or as part of a scheduled/dedicated release which enacts the freeze for everyone who updates to that version.

This freeze schedule will have some undesirable side effects, particularly for clients and servers which are slow to update or support the new endpoints. Newly uploaded images, files, avatars, etc may appear "broken" or missing to users on older software. Existing media should continue to work, however, reducing the impact from 100% of media to a smaller percentage.

Servers SHOULD consider whether their users' typical clients will break as part of the freeze before enacting the freeze. Clients SHOULD update as soon as reasonably possible to support authenticated media, particularly following the spec release containing this MSC. Other considerations may include bridges, deployment-specific use cases, and patch availability.

It is worth noting that the matrix.org homeserver plans to freeze media relatively quickly following this proposal's release in the specification. Details will be published to the matrix.org blog closer to the spec release date.

The following are specific backwards compatibility cases:

a. New clients & servers with older servers: The M_UNRECOGNIZED error behaviour should be followed to indicate that the server does not support the new endpoints, particularly when partnered with a 404 HTTP status code. Clients and servers should use the unauthenticated endpoints in this case. The endpoints will not be frozen by the server, so should work for 'new' media.

b. Older clients & servers with newer servers: Mentioned above, servers are strongly encouraged to freeze unauthenticated media access within a relatively quick timeframe. Though media before the freeze should remain accessible, clients and older federating servers may still see errors when accessing new media, leading to client UI feeling "broken" or missing avatars. The various considerations above are meant to reduce the impact of this case.
Removal of allow_redirect parameter from /download and /thumbnail

Clients MUST expect a 307 or 308 redirect when calling the new /download and /thumbnail Client-Server API endpoints.

Servers MUST expect the Location header in the media part of the new Server-Server API /download and /thumbnail endpoints. Servers MUST NOT respond with a 307 or 308 redirect at the top level for the endpoint - they can only redirect within the media part itself.

See this comment on MSC3860 for further context on this change.

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).

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).

Servers MAY flag the media as exceptions to the freeze described in part 5 of this proposal ("Backwards compatibility mechanisms"). Clients SHOULD continue to use the unauthenticated media download/thumbnail endpoints to access these icons until a future MSC can improve the situation.

The proposal author expects that the spec's transition to OIDC will ultimately replace this mechanism, or that an MSC could be opened to allow HTTP(S) URLs in place of mxc: URIs.

(This was previously discussed in MSC2858.)

Potential issues

Setting the Authorization header is particularly annoying for web clients. Service workers are seemingly the best option, though other options include locally-cached blob: URIs. Clients should note that caching media can lead to significant memory usage, particularly for large media. Service workers by comparison allow for proxy-like behaviour.

Cookies are a plausible mechanism for sharing session information between requests without having to set headers, though would be a relatively bespoke authentication method for Matrix. Additionally, many Matrix users have cookies disabled due to the advertising and tracking use cases common across the web.
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.
Over federation, the use of the Range request header on the federation endpoints becomes unclear as it could affect either or both parts of the response. There does not appear to be formal guidance in RFC 9110 either. There are arguments for affecting both and either part equally. Typically, such a header would be used to resume failed downloads, though servers are already likely to discard received data and fail the associated client requests when the federation request fails. Therefore, servers are unlikely to use Range at all. As such, this proposal does not make a determination on how Range should be handled, and leaves it as an HTTP specification interpretation problem instead.
The Location header support on the new federation endpoints could add a bit of complexity to servers, though given the alternative of supporting CDNs and similar is to place complexity into "edge workers" to mutate the response value. Though the Matrix spec would be "simpler", the edge worker setup would be fragmented where we have an opportunity for a common standard.

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, or applied after the fact by a server administrator.
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 and GET /_matrix/media/v3/create. The reason to delay doing so is because MSC3911 will make more substantial changes to these endpoints, requiring another rename, and it is expected that both proposals will be merged near to the same time as each other (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 the federation endpoints providing the info directly.

This is a plausible approach with no significant upsides or downsides when compared to multipart responses.

Similarly, custom headers could be used to carry the metadata on the response, though again, there are no significant upsides or downsides to doing so.

Readers may wish to refer to this thread on the MSC which covers the majority of the pros and cons for all 3 approaches.

Compared to MSC3796 (MSC701)

MSC701/3796 introduces a concept of "content tokens" which have authentication tie-in to prevent anonymous users from accessing media. This is a similar problem space to this proposal, though deals more in the event-to-media linking space instead. Although the MSC is an early sketch, it's unclear if the problems noted on the MSC itself are feasibly resolvable.

Compared to MSC2461

MSC2461 adds authentication to the existing media endpoints, which as noted here in the Alternatives is not likely to roll out quickly and leaves an inconsistency in the spec. MSC2461 also introduces a client-visible flag for which kinds of media may require authentication, making it similar to the alternative listed above where on the federation side we could have two endpoints: one for information and one for the media itself. MSC2461 simply makes the information client-visible instead of server-visible.

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.v2/media/download/{mediaId}
- Note: This endpoint has a .v2 in its unstable identifier due to the MSC changing after initial implementation. The original unstable endpoint has a serverName and may still be supported by some servers: GET /_matrix/federation/unstable/org.matrix.msc3916/media/download/{serverName}/{mediaId}
  
  The serverName was later dropped in favour of explicit scoping. See allow_remote details in the MSC body for details.
GET /_matrix/federation/unstable/org.matrix.msc3916.v2/media/thumbnail/{mediaId}
- Note: This endpoint has a .v2 in its unstable identifier due to the MSC changing after initial implementation. The original unstable endpoint has a serverName and may still be supported by some servers: GET /_matrix/federation/unstable/org.matrix.msc3916/media/thumbnail/{serverName}/{mediaId}
  
  The serverName was later dropped in favour of explicit scoping. See allow_remote details in the MSC body for details.

Stable flag

After the proposal is accepted servers may advertise support for the stable endpoints by setting org.matrix.msc3916.stable to true in the unstable_features section of the versions endpoint in addition to the usual version-based feature support. This option is provided to encourage a faster rollout in the wider Matrix ecosystem until servers support the full feature set of the respective version of the Matrix specification.

Dependencies

None.

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