How severe is CVE-2021-28799?

CVE-2021-28799 has a CVSS score of 10/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-28799?

Affected package information for CVE-2021-28799 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-28799?

No fixed version has been published for CVE-2021-28799 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-28799 actively exploited in the wild?

Yes. There are 3 known exploit references for CVE-2021-28799, including 3 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-28799?

CVE-2021-28799 has an EPSS (Exploit Prediction Scoring System) score of 92.4%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-28799 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-28799 (QNAP NAS Improper Authorization Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2022-03-31. CISA required all US federal civilian agencies to remediate this vulnerability by 2022-04-21. The required action was: Apply updates per vendor instructions. This CVE has been used in known ransomware campaigns. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

What type of vulnerability is CVE-2021-28799?

CVE-2021-28799 is classified as CWE-285 (CWE-285). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

Which products and versions are affected by CVE-2021-28799?

CVE-2021-28799 affects qnap hybrid backup sync. Check the affected-products list on this page against your deployed version to confirm exposure.

Is there exploit tooling available for CVE-2021-28799?

A Nuclei detection template exists for CVE-2021-28799, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-28799 published, and has it been updated?

CVE-2021-28799 was published on May 13, 2021 and was last updated on November 3, 2025. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2022-03-31 — agencies required to remediate by 2022-04-21 · Ransomware

CVE-2021-28799

CRITICAL

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…

Published

May 13, 2021

Updated

Nov 3, 2025

Affected

0 pkgs

Patched

None yet

Exploits

3 known

EPSS Exploitation Probability

via FIRST.org ↗

92.4%probability of exploitation in next 30 days

Very High Risk100th percentile+1.53%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

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 .

Affected Products

1 product · 5 configurations

Application

hybrid backup syncqnap

< 16.0.0419

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

An improper authorization vulnerability has been reported to affect QNAP…

facebook.comAPIAug 2021

An improper authorization vulnerability has been reported to affect QNAP…

cisa.govCISAMar 2022

An improper authorization vulnerability has been reported to affect QNAP…

bleepingcomputer.comAPIMay 2021

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-28799 in your stack?

O3 detects CVE-2021-28799 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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