# CVE-2021-28809: Missing Authentication for Critical Function in RTRR Server in HBS3

> Attackers can exploit missing authentication in the RTRR server to compromise the operating system security on affected QNAP devices.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Jul 2021
- **Updated:** 17 Sep 2024
- **Also known as:** GHSA-fwr8-wff7-fmxw

## What is CVE-2021-28809?

CVE-2021-28809 is a critical missing authorization vulnerability in QNAP Systems Inc. HBS 3, published on 8 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An improper access control vulnerability has been reported to affect certain legacy versions of HBS 3. If exploited, this vulnerability allows attackers to compromise the security of the operating system.QNAP have already fixed this vulnerability in the following versions of HBS 3: QTS 4.3.6: HBS 3 v3.0.210507 and later QTS 4.3.4: HBS 3 v3.0.210506 and later QTS 4.3.3: HBS 3 v3.0.210506 and later

Weakness: CWE-749 (Exposed Dangerous Method or Function), CWE-306 (Missing Authentication for Critical Functionality), CWE-284 (Improper Access Control)

## Which versions are affected by CVE-2021-28809?

It affects 1 product.


Products: Qnap Hybrid Backup Sync.

## How do I fix CVE-2021-28809?

No fix has been published for CVE-2021-28809 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-28809 being exploited?

There is no known exploitation of CVE-2021-28809 in the wild and no public exploit code.

## Other vulnerabilities in QNAP Systems Inc. HBS 3

- [CVE-2021-28799](https://insights.sbomify.com/CVE-2021-28799/): QNAP NAS Improper Authorization (critical 10.0, exploited in the wild)
- [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-2021-28809 and fix it if it is affected.

What it is: CVE-2021-28809 is a critical missing authorization vulnerability in QNAP Systems Inc. HBS 3, published on 8 Jul 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-28809 in the wild and no public exploit code.

Affected product: QNAP Systems Inc. HBS 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-2021-28809/ (Markdown: https://insights.sbomify.com/CVE-2021-28809/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-28809
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-28809
- OSV: https://osv.dev/vulnerability/CVE-2021-28809

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-28809/
Summaries are generated and may contain mistakes; the official description above is the record.