What is the CVSS score of CVE-2026-21891?

CVE-2026-21891 has a CVSS 3.1 base score of 9.4 (Critical). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L. EPSS exploitation probability: 13.6%.

Which versions of Zimaos are affected by CVE-2026-21891?

Organizations using versions face critical risk from CVE-2026-21891, a improper authentication vulnerability rated CVSS 9.4.

Is there a public PoC exploit available for CVE-2026-21891?

Yes, a public proof-of-concept exploit is available for CVE-2026-21891. Prioritise patching immediately. EPSS: 13.6%.

How to fix or mitigate CVE-2026-21891?

Within 24 hours: Identify all affected systems running versions and apply vendor patches immediately. Audit authentication configurations and rotate any potentially compromised credentials.

Zimaos CVE-2026-21891

CRITICAL

Improper Authentication (CWE-287)

2026-01-08 security-advisories@github.com

Authentication Bypass Zimaos Zimaspace

9.4

CVSS 3.1 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

9.4 CRITICAL

AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L

Primary rating from GitHub Advisory · only source for this CVE.

CVSS VectorGitHub Advisory

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

Low

Lifecycle Timeline

Analysis Generated

Mar 12, 2026 - 21:54 vuln.today

PoC Detected

Jan 12, 2026 - 17:13 vuln.today

Public exploit code

CVE Published

Jan 08, 2026 - 14:15 nvd

CRITICAL 9.4

DescriptionGitHub Advisory

ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In versions up to and including 1.5.0, the application checks the validity of the username but appears to skip, misinterpret, or incorrectly validate the password when the provided username matches a known system service account. The application's login function fails to properly handle the password validation result for these users, effectively granting authenticated access to anyone who knows one of these common usernames and provides any password. As of time of publication, no known patched versions are available.

AnalysisAI

ZimaOS (fork of CasaOS) through 1.5.0 has an authentication bypass where passwords for system service accounts are not properly validated during login. Attackers can access the system using known service account names with any password. PoC available, EPSS 13.6%.

Technical ContextAI

The login function fails to properly validate passwords when the provided username matches a known system service account (CWE-287). The application may skip, misinterpret, or incorrectly validate the password, effectively allowing passwordless login with service account names.