How severe is CVE-2021-28809?

CVE-2021-28809 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-28809 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 16% (96th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-28809?

CVE-2021-28809 affects Qnap Hybrid Backup Sync. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-28809?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-28809 published?

CVE-2021-28809 was published on 2021-07-08 and last updated on 2026-06-17.

CVE-2021-28809

CVE-2021-28809 is a critical-severity vulnerability in Qnap Hybrid Backup Sync with a CVSS 3.x base score of 9.8. Its EPSS exploit-prediction score of 16% places it in the 96th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-284.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 10.0
EPSS exploit prediction: 16% (96th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-284
Affected product: Qnap Hybrid Backup Sync
Published: 2021-07-08
Last modified: 2026-06-17

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

Frequently asked questions

What is CVE-2021-28809?: 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
How severe is CVE-2021-28809?: CVE-2021-28809 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-28809 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 16% (96th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-28809?: CVE-2021-28809 affects Qnap Hybrid Backup Sync. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-28809?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-28809 published?: CVE-2021-28809 was published on 2021-07-08 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*

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