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Vulnerability record · CVE-2021-28809 · published 8 July 2021

CVE-2021-28809: Qnap hybrid backup sync improper access control vulnerability

Qnap · Hybrid Backup Sync

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

9.8 CVSS 3.1 Critical EPSS 16% · top 3.2% CWE-284 · Improper access controlCWE-306 · Missing authentication for critical function
9.8CVSS 3.1 base score, v2 10.0
16%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
4References
17 Jun 2026Last modified by NVD

Description

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:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://www.qnap.com/en/security-advisory/qsa-21-19 Vendor Advisory
https://www.zerodayinitiative.com/advisories/ZDI-21-783/ Third Party AdvisoryVDB Entry
https://www.qnap.com/en/security-advisory/qsa-21-19 Vendor Advisory
https://www.zerodayinitiative.com/advisories/ZDI-21-783/ Third Party AdvisoryVDB Entry

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2021-28809), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.