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
10.0
EchelonGraph verdictPatch nowTreat as an emergency — this is being exploited.
  • Actively exploited in the wild (CISA-KEV)
CISA-KEV: ExploitedEPSS: 78%CVSS: 10.0Exploit: NoneExposed: 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
99.5%
KEV
⚠ Exploited

Published

May 13, 2021

Last Modified

November 3, 2025

Advisory Details (2)

Auto-updated Jul 18, 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 51× in last 7d / 211× 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 378 total refreshes for this CVE.

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

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 ↗

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

Explore →

Is Your Infrastructure Affected by CVE-2021-28799?

EchelonGraph automatically scans your cloud infrastructure and maps CVE exposure using blast radius analysis.

Start Free Scan →Browse All CVEs