CVE-2021-28799

CRITICALNVD 10.010.0—
EchelonGraph scoreHIGH confidence

Score elevated to 10.0 because this CVE is listed on the CISA Known Exploited Vulnerabilities catalog (added 2022-03-31), indicating real-world exploitation has been confirmed by US federal agencies. NVD baseline CVSS 10.0 retained for reference. Confidence: HIGH.

Triggered by: CISA KEV (actively exploited)
Sources: cisa_kev, epss, ghsa, nvd
Trending — 4 sources updated this weekExploited in the wild
10.0EG
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS PROB: 78%CVSS: 10.0Exploit: cisa kev · ssvc active · kev ransomware · epss top5pct · epss high · public exploitExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a 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 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 .

CVSS v3
10.0
EG Score
10.0(high)
EG Risk
100(Act)
EG Risk 100/100SSVC: Act

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity100% × 45%
Exploitation100% × 40%
Automatability100% × 15%
Action: Fix now — active exploitation, automatable, high impact.
EPSS PROB
78%
EPSS %ILE
100%
KEV
⚠ Exploited

Published

May 13, 2021

Last Modified

November 3, 2025

Advisory Details (2)

Auto-updated Sep 19, 2026
⚠️ Active exploitation confirmed. Patch available. Sources: cisa.
cisa Patch Available🔴 Active Exploitation

Known Exploited Vulnerabilities Catalog | CISA

Known Exploited Vulnerabilities Catalog | CISA. Listed in CISA Known Exploited Vulnerabilities catalog.

https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-28799
generic

Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync) - Security Advisory | QNAP

https://www.qnap.com/en/security-advisory/QSA-21-13

Weakness Classification(2)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 54× in last 7d / 228× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 793 total refreshes for this CVE.

  1. 2026-09-19 16:17 UTCGHSA enrichment
  2. 2026-09-19 12:05 UTCGHSA enrichment
  3. 2026-09-19 07:54 UTCGHSA enrichment
  4. 2026-09-19 03:43 UTCGHSA enrichment
  5. 2026-09-18 23:31 UTCEG score recompute
  6. 2026-09-18 23:31 UTCGHSA enrichment
  7. 2026-09-18 19:23 UTCEPSS rescore
  8. 2026-09-18 19:20 UTCGHSA enrichment
  9. 2026-09-18 19:20 UTCCISA KEV update
  10. 2026-09-18 15:08 UTCGHSA enrichment
  11. 2026-09-18 14:15 UTCCISA KEV update
  12. 2026-09-18 10:57 UTCGHSA enrichment
  13. 2026-09-18 06:46 UTCGHSA enrichment
  14. 2026-09-18 02:35 UTCGHSA enrichment
  15. 2026-09-17 22:23 UTCEG score recompute
  16. 2026-09-17 22:23 UTCGHSA enrichment
  17. 2026-09-17 19:23 UTCEPSS rescore
  18. 2026-09-17 19:23 UTCEPSS rescore
  19. 2026-09-17 18:11 UTCGHSA enrichment
  20. 2026-09-17 13:59 UTCGHSA enrichment
  21. 2026-09-17 09:48 UTCGHSA enrichment
  22. 2026-09-17 05:37 UTCGHSA enrichment
  23. 2026-09-17 01:26 UTCGHSA enrichment
  24. 2026-09-16 21:14 UTCGHSA enrichment
  25. 2026-09-16 17:02 UTCEG score recompute
Show 75 more
  1. 2026-09-16 17:02 UTCGHSA enrichment
  2. 2026-09-16 14:10 UTCCISA KEV update
  3. 2026-09-16 14:05 UTCEPSS rescore
  4. 2026-09-16 12:51 UTCGHSA enrichment
  5. 2026-09-16 08:40 UTCGHSA enrichment
  6. 2026-09-16 04:27 UTCGHSA enrichment
  7. 2026-09-16 00:15 UTCGHSA enrichment
  8. 2026-09-15 20:04 UTCGHSA enrichment
  9. 2026-09-15 15:51 UTCGHSA enrichment
  10. 2026-09-15 11:39 UTCGHSA enrichment
  11. 2026-09-15 07:28 UTCGHSA enrichment
  12. 2026-09-15 03:17 UTCEG score recompute
  13. 2026-09-15 03:17 UTCGHSA enrichment
  14. 2026-09-15 03:06 UTCEPSS rescore
  15. 2026-09-15 03:06 UTCEPSS rescore
  16. 2026-09-14 23:06 UTCGHSA enrichment
  17. 2026-09-14 19:24 UTCCISA KEV update
  18. 2026-09-14 18:55 UTCGHSA enrichment
  19. 2026-09-14 14:44 UTCGHSA enrichment
  20. 2026-09-14 10:33 UTCGHSA enrichment
  21. 2026-09-14 06:20 UTCGHSA enrichment
  22. 2026-09-14 02:09 UTCGHSA enrichment
  23. 2026-09-13 21:58 UTCGHSA enrichment
  24. 2026-09-13 17:47 UTCGHSA enrichment
  25. 2026-09-13 13:36 UTCGHSA enrichment
  26. 2026-09-13 09:24 UTCGHSA enrichment
  27. 2026-09-13 05:13 UTCGHSA enrichment
  28. 2026-09-13 01:02 UTCGHSA enrichment
  29. 2026-09-12 20:51 UTCGHSA enrichment
  30. 2026-09-12 16:40 UTCEG score recompute
  31. 2026-09-12 16:40 UTCGHSA enrichment
  32. 2026-09-12 14:57 UTCEPSS rescore
  33. 2026-09-12 12:28 UTCGHSA enrichment
  34. 2026-09-12 08:17 UTCGHSA enrichment
  35. 2026-09-12 04:06 UTCGHSA enrichment
  36. 2026-09-11 23:55 UTCGHSA enrichment
  37. 2026-09-11 19:44 UTCGHSA enrichment
  38. 2026-09-11 18:49 UTCCISA KEV update
  39. 2026-09-11 15:33 UTCGHSA enrichment
  40. 2026-09-11 11:21 UTCEG score recompute
  41. 2026-09-11 11:21 UTCGHSA enrichment
  42. 2026-09-11 09:33 UTCEPSS rescore
  43. 2026-09-11 07:06 UTCGHSA enrichment
  44. 2026-09-11 02:55 UTCGHSA enrichment
  45. 2026-09-10 22:44 UTCGHSA enrichment
  46. 2026-09-10 19:28 UTCCISA KEV update
  47. 2026-09-10 18:33 UTCGHSA enrichment
  48. 2026-09-10 14:22 UTCGHSA enrichment
  49. 2026-09-10 10:11 UTCEG score recompute
  50. 2026-09-10 10:11 UTCGHSA enrichment
  51. 2026-09-10 05:59 UTCGHSA enrichment
  52. 2026-09-10 01:45 UTCGHSA enrichment
  53. 2026-09-09 21:33 UTCGHSA enrichment
  54. 2026-09-09 19:08 UTCCISA KEV update
  55. 2026-09-09 17:22 UTCGHSA enrichment
  56. 2026-09-09 13:11 UTCGHSA enrichment
  57. 2026-09-09 08:59 UTCGHSA enrichment
  58. 2026-09-09 04:48 UTCGHSA enrichment
  59. 2026-09-09 00:36 UTCGHSA enrichment
  60. 2026-09-08 20:25 UTCGHSA enrichment
  61. 2026-09-08 18:54 UTCCISA KEV update
  62. 2026-09-08 16:12 UTCGHSA enrichment
  63. 2026-09-08 11:59 UTCGHSA enrichment
  64. 2026-09-08 07:46 UTCGHSA enrichment
  65. 2026-09-08 03:35 UTCGHSA enrichment
  66. 2026-09-07 23:23 UTCGHSA enrichment
  67. 2026-09-07 19:10 UTCGHSA enrichment
  68. 2026-09-07 14:58 UTCGHSA enrichment
  69. 2026-09-07 10:47 UTCGHSA enrichment
  70. 2026-09-07 06:34 UTCGHSA enrichment
  71. 2026-09-07 02:19 UTCGHSA enrichment
  72. 2026-09-06 22:05 UTCGHSA enrichment
  73. 2026-09-06 17:53 UTCEG score recompute
  74. 2026-09-06 17:53 UTCGHSA enrichment
  75. 2026-09-06 13:44 UTCEPSS rescore

Publicly available exploits

(1 reference)

Working exploit code is in the public domain. Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • Nucleihttp/cves/2021/CVE-2021-28799.yaml
    First seen Jan 1, 2021

    QNAP HBS 3 - Broken Access Control

    Open source ↗

Related CVEs(same CWE)

Frequently asked(6)

What is CVE-2021-28799?
CVE-2021-28799 is a critical vulnerability published on May 13, 2021. 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…
When was CVE-2021-28799 disclosed?
CVE-2021-28799 was first published in the National Vulnerability Database on May 13, 2021, with the most recent update on November 3, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-28799 actively exploited?
Yes. CISA added CVE-2021-28799 to the Known Exploited Vulnerabilities catalog on March 31, 2022, affecting QNAP Network Attached Storage (NAS). KEV listing indicates confirmed exploitation in the wild; this CVE warrants immediate patching attention.
What is the CVSS score of CVE-2021-28799?
CVE-2021-28799 has a CVSS v3 base score of 10.0 (NVD).
Which products are affected by CVE-2021-28799?
CVE-2021-28799 affects QNAP Network Attached Storage (NAS). The full affected-products list, including version ranges and fixed versions, is shown in the Affected Packages section of this page.
How do I remediate CVE-2021-28799?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-28799, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

Explore the affected products and dependency analysis for CVE-2021-28799

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