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Actively exploitedKnown ransomware use

CVE-2021-28799 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.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 31 Mar 2022.
  • CISA lists it as known to be used in ransomware campaigns.
  • Rated Critical severity (CVSS 10.0, scored by Vendor).
  • It has been in the catalogue for 1,681 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

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 .

Affected Versions

ProductVersions
QNAP Systems Inc. HBS 3unspecified to before v16.0.0415
QNAP Systems Inc. HBS 3unspecified to before v3.0.210412
QNAP Systems Inc. HBS 3unspecified to before v3.0.210411
QNAP Systems Inc. HBS 3unspecified to before v3.0.210411
QNAP Systems Inc. HBS 3unspecified to before v16.0.0419
QNAP Systems Inc. HBS 3unspecified to before v16.0.0419

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for QNAP Network Attached Storage (NAS). Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 21 Apr 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited QNAP Flaws

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All 11 exploited QNAP vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-28799
  3. CVE record: CVE-2021-28799
  4. FIRST EPSS
  5. qnap.com/en/security-advisory/QSA-21-13