How severe is CVE-2026-21891?

CVE-2026-21891 has a CVSS 3.x base score of 9.4, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability low.

Is CVE-2026-21891 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (80th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-21891?

CVE-2026-21891 affects Zimaspace Zimaos. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-21891?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-21891 have an EU (EUVD) identifier?

Yes. CVE-2026-21891 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-1670.

When was CVE-2026-21891 published?

CVE-2026-21891 was published on 2026-01-08 and last updated on 2026-06-17.

CVE-2026-21891

CVE-2026-21891 is a critical-severity vulnerability in Zimaspace Zimaos with a CVSS 3.x base score of 9.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-287.

Key facts

Severity: Critical (CVSS 3.x base score 9.4)
EPSS exploit prediction: 2% (80th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-1670
Weakness: CWE-287
Affected product: Zimaspace Zimaos
Published: 2026-01-08
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2026-21891?: 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.
How severe is CVE-2026-21891?: CVE-2026-21891 has a CVSS 3.x base score of 9.4, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability low.
Is CVE-2026-21891 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (80th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-21891?: CVE-2026-21891 affects Zimaspace Zimaos. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-21891?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-21891 have an EU (EUVD) identifier?: Yes. CVE-2026-21891 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-1670.
When was CVE-2026-21891 published?: CVE-2026-21891 was published on 2026-01-08 and last updated on 2026-06-17.

References

https://github.com/IceWhaleTech/ZimaOS/security/advisories/GHSA-xj93-qw9p-jxq4

Affected products (1)

cpe:2.3:o:zimaspace:zimaos:*:*:*:*:*:*:*:*

More vulnerabilities in Zimaspace Zimaos

CVE-2026-28798 — Critical (CVSS 9.0): ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. Prior to version 1.5.3,…
CVE-2026-28442 — High (CVSS 8.5): ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In version 1.5.2-beta3,…
CVE-2026-28286 — High (CVSS 8.5): ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In version 1.5.2-beta3,…
CVE-2025-58432 — High (CVSS 7.8): ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In version 1.4.1 and all…
CVE-2024-49359 — High (CVSS 7.5): ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In version 1.2.4 and all…
CVE-2024-49357 — High (CVSS 7.5): ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI. In version 1.2.4 and all…

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