CVE-2021-28809

Name: CVE-2021-28809
Creator: NVD / NIST
Published: 2021-07-08T08:15:07.663+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.8/10EPSS 15.80%

Last modified Jun 17, 2026

CVE-2021-28809 is a critical-severity vulnerability rated 9.8/10 on the CVSS scale. 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. EPSS estimates a 15.80% chance of exploitation in the next 30 days.

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

Metrics

CVSS 3.1

9.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Probability

15.80%

96.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Qnap	Hybrid Backup Sync	< 3.0.210507
Qnap	Hybrid Backup Sync	< 3.0.210506

References

https://www.qnap.com/en/security-advisory/qsa-21-19Vendor Advisory
https://www.zerodayinitiative.com/advisories/ZDI-21-783/Third Party Advisory, VDB Entry
https://www.qnap.com/en/security-advisory/qsa-21-19Vendor Advisory
https://www.zerodayinitiative.com/advisories/ZDI-21-783/Third Party Advisory, VDB Entry

Timeline

Published: Jul 8, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-28809?

How severe is CVE-2021-28809?

CVE-2021-28809 has a CVSS score of 9.8/10 (CRITICAL severity). The EPSS model estimates a 15.80% probability of exploitation in the next 30 days.

How do I fix CVE-2021-28809?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-28809?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST