CVE-2021-28799
CRITICALCISA KEVEPSS 100th pctlDescription
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 .
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Exploit Intelligence
Very high risk: more likely to be exploited than 100% of all known CVEs.
QNAP NAS Improper Authorization Vulnerability
QNAP NAS running HBS 3 contains an improper authorization vulnerability which can allow remote attackers to log in to a device.
Apply updates per vendor instructions.
Remediation due: 2022-04-21
Associated with ransomware campaigns
References
Related Vulnerabilities
Other CWE-285 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2025-29927 | Critical | 9.1 | 99% | - | Fix |
| CVE-2023-32707 | High | 8.8 | 79% | - | Fix |
| CVE-2023-48241 | High | 7.5 | 73% | - | Fix |
| CVE-2023-22480 | High | 7.3 | 67% | - | Fix |
| CVE-2022-3229 | Critical | 9.8 | 66% | - | - |
| CVE-2016-5676 | High | 7.5 | 54% | - | - |
Common questions
How do I fix CVE-2021-28799?
Upgrade hybrid backup sync to 16.0.0419 or later.
Is CVE-2021-28799 being actively exploited?
Yes. CVE-2021-28799 is listed in the CISA Known Exploited Vulnerabilities catalog, which means exploitation has been observed in the wild. CISA has also linked it to known ransomware campaigns. US federal agencies were required to remediate it by 2022-04-21. Treat it as a priority whatever its CVSS score says.
How severe is CVE-2021-28799?
CVE-2021-28799 has a CVSS v3 base score of 10.0, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2021-28799 affect?
Published advisories record a fix for hybrid backup sync (NVD). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
Embed a live status badge for CVE-2021-28799
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2025-11-03.