CVE-2026-27707: Seerr Authentication Bypass Vulnerability

CVE-2026-27707 Overview

CVE-2026-27707 is a critical authentication bypass vulnerability affecting Seerr, an open-source media request and discovery manager for Jellyfin, Plex, and Emby media servers. The vulnerability exists due to an authentication guard logic flaw in the POST /api/v1/auth/jellyfin endpoint that allows unauthenticated attackers to register a new Seerr account on any Plex-configured instance by authenticating with an attacker-controlled Jellyfin server.

This flaw enables attackers to gain authenticated sessions with default permissions, including the ability to submit media requests to Radarr/Sonarr integration services. The vulnerability affects Seerr versions 2.0.0 through versions prior to 3.1.0.

Critical Impact

Unauthenticated attackers can gain unauthorized access to Seerr instances configured for Plex, enabling them to submit media requests and potentially abuse connected Radarr/Sonarr services.

Affected Products

• Seerr versions 2.0.0 to versions prior to 3.1.0
• Plex-configured Seerr deployments with default Jellyfin settings
• Deployments where settings.main.newPlexLogin is set to true (default)

Discovery Timeline

• 2026-02-27 - CVE-2026-27707 published to NVD
• 2026-03-04 - Last updated in NVD database

Technical Details for CVE-2026-27707

Vulnerability Analysis

The vulnerability stems from insufficient validation in the authentication guard logic for the Jellyfin authentication endpoint. When processing authentication requests at POST /api/v1/auth/jellyfin, the application fails to properly verify whether the Jellyfin server being used for authentication is the legitimately configured server for the Seerr instance.

This allows an attacker to bypass authentication by pointing the authentication request to an attacker-controlled Jellyfin server. Since the application trusts the response from any Jellyfin server without validating its legitimacy, an attacker can successfully authenticate and receive a valid session token.

The vulnerability is classified under CWE-288 (Authentication Bypass Using an Alternate Path or Channel), which describes security mechanisms that can be circumvented through alternative authentication methods that were not intended to grant access.

Root Cause

The root cause lies in the authentication flow's failure to verify the source Jellyfin server against the configured server. Specifically, when all three of the following conditions are true, the vulnerability is exploitable:

1. settings.main.mediaServerType is set to PLEX (the most common deployment configuration)
2. settings.jellyfin.ip is set to "" (default empty value, meaning Jellyfin was never configured)
3. settings.main.newPlexLogin is set to true (default setting)

This combination of default settings creates a scenario where the authentication endpoint accepts credentials from any Jellyfin server, effectively bypassing the intended authentication mechanism.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker exploits this vulnerability by:

1. Setting up a malicious Jellyfin server under their control
2. Sending a crafted authentication request to the target Seerr instance's /api/v1/auth/jellyfin endpoint
3. Pointing the authentication to their malicious Jellyfin server
4. Receiving a valid authenticated session from Seerr

The attacker-controlled Jellyfin server validates the attacker's credentials, and Seerr trusts this validation, granting the attacker an authenticated session with default permissions. This enables immediate access to application features including media request submission to integrated Radarr/Sonarr services.

Jellyfin-configured and Emby-configured deployments are not affected by this vulnerability. For detailed exploitation mechanics, refer to the GitHub Security Advisory GHSA-rc4w-7m3r-c2f7.

Detection Methods for CVE-2026-27707

Indicators of Compromise

• Unexpected POST requests to /api/v1/auth/jellyfin from external IP addresses
• New user account registrations with Jellyfin authentication on Plex-configured instances
• Authentication logs showing Jellyfin server IPs that do not match your infrastructure
• Unexpected media requests submitted to Radarr/Sonarr integrations

Detection Strategies

• Monitor authentication logs for /api/v1/auth/jellyfin endpoint activity on Plex-configured instances
• Implement alerting for new user registrations that authenticate via Jellyfin when Jellyfin is not configured
• Review network traffic for connections to unknown Jellyfin server IP addresses during authentication flows
• Audit existing user accounts for any unauthorized registrations created through Jellyfin authentication

Monitoring Recommendations

• Enable verbose logging for authentication events in Seerr
• Configure network monitoring to detect outbound connections from Seerr to unexpected Jellyfin servers
• Set up alerts for any new user creation events, particularly those authenticated via Jellyfin
• Review Radarr/Sonarr request logs for unusual media request patterns from newly created accounts

How to Mitigate CVE-2026-27707

Immediate Actions Required

• Upgrade Seerr to version 3.1.0 or later immediately
• Audit existing user accounts for any unauthorized registrations
• Review Radarr/Sonarr request history for suspicious activity
• If upgrade is not immediately possible, implement the workarounds below

Patch Information

The vulnerability has been fixed in Seerr version 3.1.0. The fix addresses the authentication guard logic flaw by properly validating Jellyfin server configurations before accepting authentication requests. Administrators should upgrade to the patched version as soon as possible.

Relevant security resources:

• GitHub Security Advisory GHSA-rc4w-7m3r-c2f7
• Patch Commit Details
• Seerr Release v3.1.0

Workarounds

• Disable new Plex logins by setting settings.main.newPlexLogin to false until the upgrade is applied
• Configure a valid Jellyfin IP address in settings.jellyfin.ip to prevent exploitation via empty default value
• Implement network-level access controls to restrict access to the /api/v1/auth/jellyfin endpoint
• Place Seerr behind an authentication proxy that requires valid credentials before reaching the application

# Verify current Seerr version and check for vulnerability
# If running version 2.0.0 to <3.1.0, upgrade immediately
docker pull ghcr.io/seerr-team/seerr:3.1.0

# Alternatively, if using Docker Compose, update your compose file
# and restart the service
docker-compose pull seerr
docker-compose up -d seerr