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CVE Record

CVE-2026-27707: Plex-configured Seerr instances vulnerable to unauthenticated account registration via Jellyfin authentication endpoint

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.

HighCVSS 7.3Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2026-27707 lets an unauthenticated outsider create a Seerr account on certain Plex-configured Seerr deployments. The new account gets normal default access, including media request capability. This is not a full server takeover based on the sources, but it weakens access control and can create unwanted application use and downstream requests.

Executive priority

Prioritize remediation for any internet-accessible Plex-based Seerr deployment. The fix is available in Seerr 3.1.0, and the vulnerable defaults may be common. Treat as a high-priority application access-control issue, especially where media request automation affects downstream systems.

Technical view

Seerr versions 2.0.0 through before 3.1.0 have an authentication guard logic flaw in POST /api/v1/auth/jellyfin. On Plex-configured instances with default unset Jellyfin IP and new Plex login enabled, an attacker can authenticate against an attacker-controlled Jellyfin server and receive a Seerr session. CVSS is 7.3 high; CWEs include CWE-288 and CWE-807.

Likely exposure

Exposure is limited to Seerr deployments running >=2.0.0 and <3.1.0 where mediaServerType is PLEX, settings.jellyfin.ip is empty, and newPlexLogin is true. Jellyfin-configured and Emby-configured deployments are reported not affected. Internet-exposed Seerr instances would carry higher practical risk.

Exploitation context

The source bundle does not report active exploitation, and KEV is false. The issue is network-reachable, requires no prior account, and no user interaction. Successful abuse creates an authenticated application session with default permissions rather than documented administrative control.

Researcher notes

The advisory’s affected-condition set is important: Plex mode plus default empty Jellyfin configuration plus new Plex login enabled. Analysis should avoid over-scoping to Jellyfin or Emby deployments, which the source says are not affected. The public sources identify the endpoint and logic class but do not provide evidence of exploitation in the wild.

Mitigation direction

  • Upgrade Seerr to version 3.1.0 or later.
  • Review the GitHub security advisory for vendor guidance.
  • If not upgrading immediately, restrict network access to Seerr.
  • Review Seerr user accounts for unexpected registrations.
  • Confirm Plex, Jellyfin, and login settings match intended configuration.

Validation and detection

  • Inventory Seerr instances and record running versions.
  • Check whether the instance version is >=2.0.0 and <3.1.0.
  • Verify mediaServerType is set to PLEX.
  • Verify settings.jellyfin.ip is empty or unset.
  • Verify settings.main.newPlexLogin is true.
  • Confirm remediation by validating Seerr is upgraded to 3.1.0 or later.
Prepared
Confidence
high
Sources
5

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-288: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · low confidence lookup

CWE-807: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-27707 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
High
CVSS
7.3 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
4Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.3CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L3.93.4Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

7.3High
CVSS 3.1 vector shape for CVE-2026-27707Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
seerr-teamseerr>= 2.0.0, < 3.1.0Listed
Weakness

CWE details

CWE-288CWE-807

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.