CVE-2021-28799

Name: CVE-2021-28799
Creator: NVD / NIST
Published: 2021-05-13T03:15:06.843+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.8/10Actively ExploitedEPSS 78.40%

Last modified Jun 17, 2026

CVE-2021-28799 is a critical-severity vulnerability rated 9.8/10 on the CVSS scale. 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. CISA has confirmed active exploitation in the wild. EPSS estimates a 78.40% chance of exploitation in the next 30 days.

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 .

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

78.40%

99.5th percentile

Probability of exploitation in the next 30 days. Learn more

Exploitation Status

This vulnerability is listed in CISA’s Known Exploited Vulnerabilities catalog, confirming active exploitation in the wild. Federal agencies must remediate by Apr 21, 2022.

Weakness Enumeration

CWE-285

Affected Software

Vendor	Product	Versions
Qnap	Hybrid Backup Sync	< 16.0.0415
Qnap	Hybrid Backup Sync	< 3.0.210412
Qnap	Hybrid Backup Sync	< 3.0.210411
Qnap	Hybrid Backup Sync	< 16.0.0419

References

https://www.qnap.com/en/security-advisory/QSA-21-13Vendor Advisory
https://www.qnap.com/en/security-advisory/QSA-21-13Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-28799US Government Resource

Timeline

Published: May 13, 2021
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2021-28799?

How severe is CVE-2021-28799?

CVE-2021-28799 has a CVSS score of 9.8/10 (CRITICAL severity). The EPSS model estimates a 78.40% probability of exploitation in the next 30 days. This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog.

How do I fix CVE-2021-28799?

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-2021-28799?

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

Scan your code now

Source: NVD / NIST