CVE-2026-21891
CriticalExploitedPublic exploitZimaOS has Authentication Bypass via System-Level Username
Published Jan 8, 2026 · Updated Aug 12, 2026 · Also known as GHSA-xj93-qw9p-jxq4
Key Facts
Patch immediately- CVE-2026-21891 is a critical-severity vulnerability (CVSS 9.8 v3.1) affecting ZimaOS (Zimaspace).
- It is exploited in the wild, listed in Industry KEV feed, CIRCL KEV since Mar 7, 2026. Public exploit code is available.
- EPSS estimates a 2.3% probability of exploitation in the next 30 days.
- Fixed versions are available, including ZimaOS <=1.5.0, ZimaOS 1.5.0.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
9.8
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS (30-day exploit likelihood)
2.3%
Higher than 83.1% of all scored CVEs
Exploited in the wild
Yes
2 catalog listings
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2026-21891 brief as a PDF
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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.
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| Industry KEV feed | Mar 7, 2026 | n/a | Not reported |
| CIRCL KEV | Mar 7, 2026 | n/a | Not reported |
Required action: Apply remediations or mitigations per vendor instructions or discontinue use of the product if remediation or mitigations are unavailable.
Affected Products
2 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| ZimaOSZimaspace | >= <= 1.5.0 | 1.5.0 |
Patched Releases
ZimaOS
<=1.5.0
Timeline
Publicly disclosed
Jan 8, 2026
Added to Industry KEV feed
Mar 7, 2026
Added to CIRCL KEV
Mar 7, 2026
Last updated
Aug 12, 2026
Weakness Type
- CWE-287 Improper Authentication
References
- advisoryhttps://github.com/CVEProject/cvelistV5/tree/main/cves/2026/21xxx/CVE-2026-21891.json
- advisoryhttps://nvd.nist.gov/vuln/detail/CVE-2026-21891
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
FAQ
Common Questions.
ZimaOS is a fork of CasaOS, an operating system for Zima devices and x86-64 systems with UEFI.
Affected: ZimaOS (Zimaspace) versions >= <= 1.5.0. The full list is in the CVE brief.
It is exploited in the wild, listed in Industry KEV feed, CIRCL KEV since Mar 7, 2026.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
Public exploit code is available.
Fixed versions are available, including ZimaOS <=1.5.0, ZimaOS 1.5.0.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
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