Nextcloud News: Authenticated blind SSRF via feed URL

Nextcloud News is an RSS/Atom feed reader. Prior to 28.3.0-beta.1, Nextcloud News allows authenticated users to add feeds by providing a feed URL (via the web interface or the API). In affected versions, an authenticated attacker could provide a URL pointing to internal/private IP ranges or localhost, causing the Nextcloud server to perform server-side HTTP requests to attacker-controlled destinations, but not relaying the result. This enables blind SSRF, which can be used to scan or probe internal network services that are reachable from the Nextcloud server. This vulnerability is fixed in 28.3.0-beta.1.