Seerr is an open-source media request and discovery manager for Jellyfin, Plex, and Emby. Starting in version 2.0.0 and prior to version 3.1.0, an authentication guard logic flaw in "POST /api/v1/auth/jellyfin" allows an unauthenticated attacker to register a new Seerr account on any Plex-configured instance by authenticating with an attacker-controlled Jellyfin server. The attacker receives an authenticated session and can immediately use the application with default permissions, including the ability to submit media requests to Radarr/Sonarr. Any Seerr deployment where all three of the following are true may be vulnerable: "settings.main.mediaServerType" is set to "PLEX" (the most common deployment).; "settings.jellyfin.ip" is set to """" (default, meaning Jellyfin was never configured); and "settings.main.newPlexLogin" is set to "true" (default). Jellyfin-configured and Emby-configured deployments are not affected. Version 3.1.0 of Seerr fixes this issue.