How severe is CVE-2021-28799?

CVE-2021-28799 has a CVSS 3.x base score of 10.0, 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-2021-28799 being actively exploited?

Yes. CVE-2021-28799 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2022-03-31, which means active exploitation has been confirmed. It should be prioritised for remediation.

What products are affected by CVE-2021-28799?

CVE-2021-28799 affects Qnap Hybrid Backup Sync. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-28799?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.

Does CVE-2021-28799 have an EU (EUVD) identifier?

Yes. CVE-2021-28799 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-15455. It is also flagged as exploited in the EUVD (since 2022-03-31).

When was CVE-2021-28799 published?

CVE-2021-28799 was published on 2021-05-13 and last updated on 2026-06-17.

CVE-2021-28799

CVE-2021-28799 is a critical-severity vulnerability in Qnap Hybrid Backup Sync with a CVSS 3.x base score of 10.0. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming it has been exploited in the wild (added 2022-03-31). The underlying weakness is classified as CWE-285.

Key facts

Severity: Critical (CVSS 3.x base score 10.0)
CVSS v2: 7.5
EPSS exploit prediction: 78% (100th percentile)
Actively exploited: Yes — listed in CISA KEV (added 2022-03-31)
EU (EUVD) id: EUVD-2021-15455
EU exploitation: Flagged exploited in the ENISA EU Vulnerability Database (since 2022-03-31)
Weakness: CWE-285
Affected product: Qnap Hybrid Backup Sync
Published: 2021-05-13
Last modified: 2026-06-17

Description

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 .

Frequently asked questions

What is CVE-2021-28799?: 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 .
How severe is CVE-2021-28799?: CVE-2021-28799 has a CVSS 3.x base score of 10.0, 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-2021-28799 being actively exploited?: Yes. CVE-2021-28799 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2022-03-31, which means active exploitation has been confirmed. It should be prioritised for remediation.
What products are affected by CVE-2021-28799?: CVE-2021-28799 affects Qnap Hybrid Backup Sync. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-28799?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.
Does CVE-2021-28799 have an EU (EUVD) identifier?: Yes. CVE-2021-28799 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2021-15455. It is also flagged as exploited in the EUVD (since 2022-03-31).
When was CVE-2021-28799 published?: CVE-2021-28799 was published on 2021-05-13 and last updated on 2026-06-17.

References

Affected products (1)

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

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