Seerr's authentication guard logic flaw in POST /api/v1/auth/jellyfin permits an attacker who can communicate with the server to create a new user account without prior authentication. The attacker then obtains a valid session cookie and immediately gains access to the application with default permissions, which includes the ability to submit media requests to backend tools such as Radarr and Sonarr. This flaw effectively enables an unauthenticated user to create privileged accounts and perform operations that normally require legitimate user credentials.

Vulnerable Seerr deployments are those running version 2.0.0 up to, but not including, 3.1.0 on a Plex-configured environment. The conditions that enable exploitation are: the mediaServerType setting is PLEX, the jellyfin.ip configuration remains empty (indicating that Jellyfin has never been set up), and the newPlexLogin flag is left at its default true value. Deployments configured for Jellyfin or Emby are not affected, and Seerr version 3.1.0 onward contains the patch that resolves the issue.

The CVSS score of 7.3 indicates a high severity vulnerability, but the EPSS score of less than 1% suggests a low probability of exploitation in the wild. The vulnerability has not been listed in CISA's KEV catalog. An affected system that is accessible from the public internet could be abused by an attacker who sends a crafted request to the /api/v1/auth/jellyfin endpoint using a malicious Jellyfin server address. The attacker does not need any prior authentication and only requires network reachability to the Seerr instance. Based on the description, it is inferred that the attack vector is remote network access, and that the attacker can exploit the flaw without additional privileges.