Matrix Media Repo (MMR) is a highly configurable multi-homeserver media repository for Matrix. Matrix Media Repo (MMR) is vulnerable to server-side request forgery, serving content from a private network it can access, under certain conditions. This is fixed in MMR v1.3.8. Users are advised to upgrade. Restricting which hosts MMR is allowed to contact via (local) firewall rules or a transparent proxy and may provide a workaround for users unable to upgrade.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.