CVE-2021-28809

CRITICALNVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.8 under NVD CVSS v3. EPSS exploit probability: 0.6%, top 31% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
Elevated
9.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 16%CVSS: 9.8Exploit: NoneExposed: 0

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

An improper access control vulnerability has been reported to affect certain legacy versions of HBS 3. If exploited, this vulnerability allows attackers to compromise the security of the operating system.QNAP have already fixed this vulnerability in the following versions of HBS 3: QTS 4.3.6: HBS 3 v3.0.210507 and later QTS 4.3.4: HBS 3 v3.0.210506 and later QTS 4.3.3: HBS 3 v3.0.210506 and later

CVSS v3
9.8
EG Score
9.8(medium)
EPSS
96.5%
KEV
Not listed

Published

July 8, 2021

Last Modified

November 21, 2024

Advisory Details (2)

Auto-updated Jun 13, 2026
No patch confirmed yet.
generic

ZDI-21-783 - TrendAI™ Zero Day Initiative™ (ZDI)

https://www.zerodayinitiative.com/advisories/ZDI-21-783/
generic

Improper Access Control Vulnerability in Legacy HBS 3 (Hybrid Backup Sync) - Security Advisory | QNAP

https://www.qnap.com/en/security-advisory/qsa-21-19

Weakness Classification(3)

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

Data Freshness Timeline

(refreshed 11× in last 7d / 43× 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.

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

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Frequently asked(5)

What is CVE-2021-28809?
CVE-2021-28809 is a critical vulnerability published on July 8, 2021. An improper access control vulnerability has been reported to affect certain legacy versions of HBS 3. If exploited, this vulnerability allows attackers to compromise the security of the operating system.QNAP have already fixed this vulnerability in the following versions of HBS 3: QTS 4.3.6: HBS 3…
When was CVE-2021-28809 disclosed?
CVE-2021-28809 was first published in the National Vulnerability Database on July 8, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-28809 actively exploited?
CVE-2021-28809 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 96.5% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-28809?
CVE-2021-28809 has a CVSS v3 base score of 9.8 (NVD).
How do I remediate CVE-2021-28809?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-28809, 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.

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