CVE-2021-28799: QNAP NAS Improper Authorization Vulnerability

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 31 Mar 2022, 322 days after the CVE record was published.

How likely is exploitation in the next 30 days?

78.3%

EPSS score, percentile 99.6 of all scored records

How severe?

10.0 Critical

CVSS v3.1 base score, given by the CNA

What should I do?

CISA deadline 21 Apr 2022. Apply updates per vendor instructions.

The CVE record links no vendor advisory, patch or mitigation.

Which versions are affected?

Affected

• before v16.0.0415
• before v3.0.210412
• before v3.0.210411
• before v3.0.210411
• before v16.0.0419
• before v16.0.0419

Unaffected all versions; all versions

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

CISA due date for this action: 21 April 2022.

Solution (from the CVE record)

QNAP have already fixed this vulnerability in the following versions of HBS 3: QTS 4.5.2: HBS 3 v16.0.0415 and later QTS 4.3.6: HBS 3 v3.0.210412 and later QTS 4.3.3 and 4.3.4: HBS 3 v3.0.210411 and later QuTS hero h4.5.1: HBS 3 v16.0.0419 and later QuTScloud c4.5.1~c4.5.4: HBS 3 v16.0.0419 and later

Advisories and patches

The CVE record links no vendor advisory, patch or mitigation.

Linked from CISA's catalog entry

• https://nvd.nist.gov/vuln/detail/CVE-2021-28799

Affected products and versions

As listed in the CVE record.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this vulnerability

QNAP NAS running HBS 3 contains an improper authorization vulnerability which can allow remote attackers to log in to a device.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

• CISA added CVE-2021-28799 to its Known Exploited Vulnerabilities (KEV) catalog on 31 Mar 2022.
• CISA marks it as known to be used in ransomware campaigns.
• The due date CISA sets for the required action is 21 Apr 2022.
• The CVE record was published on 13 May 2021, 322 days before it was added to KEV.
• The CVE record carries a CVSS v3.1 base score of 10.0 (Critical), given by its CNA.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 78.3% (percentile 99.6), as of 28 Oct 2026.
• CISA's SSVC assessment in the CVE record: exploitation active, automatable yes, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)

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 .

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

• CWE-285: Improper Authorization

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

References

• qnap.com

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