CVE-2021-28799
CriticalExploitedPublic exploitAn improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync.
Published May 13, 2021 · Updated Nov 3, 2025
Key Facts
Patch immediately- CVE-2021-28799 is a critical-severity vulnerability (CVSS 9.8 v3.1) affecting Hybrid Backup Sync (Qnap), HBS 1.3, HBS 2 and 1 more product.
- It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Apr 29, 2021 and is known to be used in ransomware campaigns. Public exploit code is available.
- EPSS estimates a 78% probability of exploitation in the next 30 days.
- Fixed versions are available, including HBS 3 v16.0.0415, HBS 3 v16.0.0419, HBS 3 v3.0.210411.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
9.8
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS (30-day exploit likelihood)
78%
Higher than 99.5% of all scored CVEs
Exploited in the wild
Yes
4 catalog listings, used by ransomware
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2021-28799 brief as a PDF
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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 .
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| CISA KEV | Mar 31, 2022 | Apr 21, 2022 | Known |
| Industry KEV feed | Apr 29, 2021 | Apr 21, 2022 | Known |
| CIRCL KEV | Mar 31, 2022 | Apr 21, 2022 | Known |
| ENISA EUVD | Mar 31, 2022 | n/a | Not reported |
Required action: Apply updates per vendor instructions.
Affected Products
4 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| Hybrid Backup SyncQnap | See advisory | 16.0.0415 |
| HBS 1.3 | See advisory | n/a |
| HBS 2 | See advisory | n/a |
| HBS 3 | >= unspecified | v16.0.0415 |
Patched Releases
HBS 3
v16.0.0415, v16.0.0419, v3.0.210411, v3.0.210412
Hybrid Backup Sync
16.0.0415
Timeline
Added to Industry KEV feed
Apr 29, 2021
Publicly disclosed
May 13, 2021
Added to CISA KEV
Mar 31, 2022
Added to CIRCL KEV
Mar 31, 2022
Added to ENISA EUVD
Mar 31, 2022
Last updated
Nov 3, 2025
Weakness Type
- CWE-285 Improper Authorization
References
- advisoryhttps://www.qnap.com/en/security-advisory/QSA-21-13
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
FAQ
Common Questions.
An improper authorization vulnerability has been reported to affect QNAP NAS running HBS 3 (Hybrid Backup Sync.
Affected: Hybrid Backup Sync (Qnap); HBS 1.3; HBS 2, among others. The full list is in the CVE brief.
It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Apr 29, 2021 and is known to be used in ransomware campaigns.
Yes. It was added on Mar 31, 2022 with a federal remediation due date of Apr 21, 2022.
Public exploit code is available.
Fixed versions are available, including HBS 3 v16.0.0415, HBS 3 v16.0.0419, HBS 3 v3.0.210411.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
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