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 patched version of the affected component has shipped for CVE-2021-28799 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.

How do I detect CVE-2021-28799 in my code and dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for the affected component. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2021-28799 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2021-28799?

O3 pinpoints whether CVE-2021-28799 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

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 78.3%, 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 Improper Authorization (CWE-285). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

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 October 21, 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

CVE-2021-28799 is a critical-severity (CVSS 10) Improper Authorization vulnerability. It is in CISA's Known Exploited Vulnerabilities catalog (added 2022-03-31) and has been used in ransomware campaigns — treat it as actively exploited and patch now. O3 Security confirms whether CVE-2021-28799 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

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

Oct 21, 2025

Affected

0 pkgs

Patched

None yet

Exploits

3 known

EPSS Exploitation Probability

via FIRST.org ↗

78.3%probability of exploitation in next 30 days

Very High Risk100th percentile-0.15%

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 .

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

Detection & mitigation playbook

Vulnerability

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for the affected component. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Remediation status
No patched version of the affected component has shipped for CVE-2021-28799 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Mitigate without a patch
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-28799 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2021-28799. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-28799 in your dependencies?

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.

Scan my dependencies How O3 SCA works

CVE-2021-28799

EPSS Exploitation Probability

Description

Detection & mitigation playbook

Detect

Remediation status

Mitigate without a patch

How O3 protects you

Frequently Asked Questions

Is CVE-2021-28799 in your dependencies?