# CVE-2026-21891: ZimaOS has Authentication Bypass via System-Level Username

> Attackers can bypass authentication in ZimaOS by using known system service usernames with any password, gaining unauthorized access.

- **Severity:** Critical 9.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Jan 2026
- **Updated:** 8 Jan 2026
- **Also known as:** GHSA-xj93-qw9p-jxq4

## What is CVE-2026-21891?

CVE-2026-21891 is a critical authentication bypass vulnerability in IceWhaleTech ZimaOS, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-21891?

It affects 1 product.


Products: Zimaspace Zimaos.

## How do I fix CVE-2026-21891?

No fix has been published for CVE-2026-21891 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21891 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21891 exists.

Public exploit code:

- ZimaOS - Authentication Bypass (nuclei): http/cves/2026/CVE-2026-21891.yaml

## Other vulnerabilities in IceWhaleTech ZimaOS

- [CVE-2026-28798](https://insights.sbomify.com/CVE-2026-28798/): Arbitrary internal service access via /v1/sys/proxy when Cloudflare Tunnel is enabled on ZimaOS (critical 9.1)
- [CVE-2026-28442](https://insights.sbomify.com/CVE-2026-28442/): ZimaOS: Arbitrary Deletion of Internal System Files via API Path Manipulation (high 8.6)
- [CVE-2026-28286](https://insights.sbomify.com/CVE-2026-28286/): ZimaOS: Unauthorized Creation of Files/Folders in Restricted System Directories via API (high 8.6)
- [CVE-2024-48931](https://insights.sbomify.com/CVE-2024-48931/): ZimaOS Arbitrary File Read via Parameter Manipulation (high 7.5)
- [CVE-2024-49357](https://insights.sbomify.com/CVE-2024-49357/): ZimaOS (Installed Applications and System Information) has Unauthorized Sensitive Data Leak (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21891 and fix it if it is affected.

What it is: CVE-2026-21891 is a critical authentication bypass vulnerability in IceWhaleTech ZimaOS, published on 8 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21891 exists.

Affected product: IceWhaleTech ZimaOS. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-21891/ (Markdown: https://insights.sbomify.com/CVE-2026-21891/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21891
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21891
- OSV: https://osv.dev/vulnerability/CVE-2026-21891

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21891/
Summaries are generated and may contain mistakes; the official description above is the record.