# CVE-2021-28799: QNAP NAS Improper Authorization

> Remote attackers can bypass authorization to log in to QNAP NAS devices running HBS 3, leading to unauthorized access.

- **Severity:** Critical 10.0 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Yes, since 31 Mar 2022 (CISA KEV), used in ransomware
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 13 May 2021
- **Updated:** 21 Oct 2025
- **Also known as:** GHSA-4v9x-j7pr-8wxq

## What is CVE-2021-28799?

CVE-2021-28799 is a critical missing authorization vulnerability in QNAP Systems Inc. HBS 1.3, published on 13 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync. ) If exploited, the vulnerability allows remote attackers to log in to a device. This issue affects: QNAP Systems Inc. HBS 3 versions prior to v16.0.0415 on QTS 4.5.2; versions prior to v3.0.210412 on QTS 4.3.6; versions prior to v3.0.210411 on QTS 4.3.4; versions prior to v3.0.210411 on QTS 4.3.3; versions prior to v16.0.0419 on QuTS hero h4.5.1; versions prior to v16.0.0419 on QuTScloud c4.5.1~c4.5.4. This issue does not affect: QNAP Systems Inc. HBS 2 . QNAP Systems Inc. HBS 1.3 .

Weakness: CWE-285 (Improper Authorization)

## Which versions are affected by CVE-2021-28799?

It affects 1 product.


Products: Qnap Hybrid Backup Sync.

## How do I fix CVE-2021-28799?

No fix has been published for CVE-2021-28799 yet. Limit exposure of the affected software until one is available. US federal agencies must fix it by 21 Apr 2022.

## Is CVE-2021-28799 being exploited?

Yes. Attackers have exploited CVE-2021-28799 in the wild since at least 31 Mar 2022, including in ransomware campaigns.

Public exploit code:

- QNAP HBS 3 - Broken Access Control (nuclei): http/cves/2021/CVE-2021-28799.yaml

## Other vulnerabilities in QNAP Systems Inc. HBS 1.3

- [CVE-2021-28809](https://insights.sbomify.com/CVE-2021-28809/): Missing Authentication for Critical Function in RTRR Server in HBS3 (critical 9.8)
- [CVE-2024-50388](https://insights.sbomify.com/CVE-2024-50388/): HBS 3 Hybrid Backup Sync (critical 9.5)
- [CVE-2025-62840](https://insights.sbomify.com/CVE-2025-62840/): HBS 3 Hybrid Backup Sync (high 7.0)
- [CVE-2025-62842](https://insights.sbomify.com/CVE-2025-62842/): HBS 3 Hybrid Backup Sync (high 7.0)
- [CVE-2024-53695](https://insights.sbomify.com/CVE-2024-53695/): HBS 3 Hybrid Backup Sync (medium 6.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-28799 and fix it if it is affected.

What it is: CVE-2021-28799 is a critical missing authorization vulnerability in QNAP Systems Inc. HBS 1.3, published on 13 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: Yes. Attackers have exploited CVE-2021-28799 in the wild since at least 31 Mar 2022, including in ransomware campaigns.

Affected product: QNAP Systems Inc. HBS 1.3. 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-2021-28799/ (Markdown: https://insights.sbomify.com/CVE-2021-28799/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-28799
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-28799
- OSV: https://osv.dev/vulnerability/CVE-2021-28799

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-28799/
Summaries are generated and may contain mistakes; the official description above is the record.