Prepare for authenticated media freeze (#17433)

As part of the rollout of
[MSC3916](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3916-authentication-for-media.md)
this PR adds support for designating authenticated media and ensuring
that authenticated media is not served over unauthenticated endpoints.

1 files changed, 12 insertions, 0 deletions

diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md

index 38b24b5044..e8bc2df798 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -1864,6 +1864,18 @@ federation_rr_transactions_per_room_per_second: 40
 Config options related to Synapse's media store.
 
 ---
+### `enable_authenticated_media`
+
+When set to true, all subsequent media uploads will be marked as authenticated, and will not be available over legacy
+unauthenticated media endpoints (`/_matrix/media/(r0|v3|v1)/download` and `/_matrix/media/(r0|v3|v1)/thumbnail`) - requests for authenticated media over these endpoints will result in a 404. All media, including authenticated media, will be available over the authenticated media endpoints `_matrix/client/v1/media/download` and `_matrix/client/v1/media/thumbnail`. Media uploaded prior to setting this option to true will still be available over the legacy endpoints. Note if the setting is switched to false
+after enabling, media marked as authenticated will be available over legacy endpoints. Defaults to false, but
+this will change to true in a future Synapse release.
+
+Example configuration:
+```yaml
+enable_authenticated_media: true
+```
+---
 ### `enable_media_repo`
 
 Enable the media store service in the Synapse master. Defaults to true.