Security Resilience

CVE-2021-28799

Access Control in Qnap Hybrid Backup Sync ≤ 16.0.0415

CISA KEVActive ExploitationEUVD ExploitedRansomware-linkedAccess Control
Published
13 May 2021
Modified
03 November 2025
KEV Added
31 March 2022
Patch / advisory
CVSS Score v3.1 10.0
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
EPSS Score 0.78 99.6th percentile
Risk Priority 100 floored blend · peak EPSS

CVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.

Summary

CVE-2021-28799 is a critical-severity Improper Authorization (CWE-285) vulnerability in Qnap Hybrid Backup Sync. Its CVSS base score is 10.0 (Critical).

Operationally, ranked in the top 0.4% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

An improper authorization vulnerability affects QNAP NAS devices running HBS 3 (Hybrid Backup Sync) versions prior to v16.0.0415 on QTS 4.5.2, prior to v3.0.210412 on QTS 4.3.6, prior to v3.0.210411 on QTS 4.3.4 and 4.3.3, and prior to v16.0.0419 on QuTS hero h4.5.1 and QuTScloud c4.5.1 through c4.5.4. The flaw is absent from HBS 2 and HBS 1.3. Successful exploitation permits remote attackers to authenticate to the device without valid credentials.

Attackers require no authentication or user interaction and can achieve full device login over the network, resulting in complete compromise of confidentiality, integrity, and availability as reflected in the CVSS 10.0 score.

QNAP has published advisory QSA-21-13 detailing the affected builds and corrective updates. The vulnerability also appears in CISA's catalog of known exploited vulnerabilities, confirming observed in-the-wild activity.

OWASP Top 10 for Web (2025)

EU & UK References

Vulnerability Data

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…

more

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 .

CWE(s)
KEV Date Added
31 March 2022

Related Threats

CVEs Like This One

CVE-2020-2509Same product: Qnap Qtsboth on KEV
CVE-2023-45028Same product: Qnap Qts
CVE-2023-47566Same product: Qnap Qts
CVE-2023-32972Same product: Qnap Qts
CVE-2018-19953Same product: Qnap Qtsboth on KEV
CVE-2023-45035Same product: Qnap Qts
CVE-2018-19943Same product: Qnap Qtsboth on KEV
CVE-2024-32766Same product: Qnap Qts
CVE-2023-32973Same product: Qnap Qts
CVE-2023-41292Same product: Qnap Qts

Affected Assets

qnap
hybrid backup sync
≤ 16.0.0415 · ≤ 3.0.210412 · ≤ 3.0.210411

Mitigating Controls

Likely Mitigating Controls AI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

addresses: CWE-285

Documented procedures facilitate correct implementation and ongoing management of authorization decisions.

addresses: CWE-285

Periodic reviews identify and correct flaws in authorization decisions or enforcement.

addresses: CWE-285

The control's documentation requirement reduces improper authorization by ensuring only mission-justified actions bypass authentication.

addresses: CWE-285

Establishing permitted attributes and values, plus auditing changes, ensures authorization decisions are based on correctly managed policy data.

addresses: CWE-285

Explicitly mandates authorizing remote access types before permitting connections, directly mitigating improper authorization.

addresses: CWE-285

The control explicitly requires authorization of each wireless access type prior to permitting connections.

addresses: CWE-285

Mandating explicit authorization of mobile device connections reduces the risk of improper authorization decisions for system access.

addresses: CWE-285

Specifying access authorizations for each account and requiring approvals for account requests enforces proper authorization decisions.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

blocks

A.5.1 only requires the existence of an overarching policy and topic-specific policies (including on access control); it does not itself perform or enforce any authorization check, so the CWE-285 weakness can still be exploited.

blocks

Mapping access rights to information classification and business requirements forces correct enforcement of authorization decisions, blocking the incorrect authorization weakness at the policy and implementation stage.

finds

Independent reviewers evaluate whether authorization logic matches policy, thereby reducing the window in which incorrect or missing authorization checks remain in production.

finds

By requiring managers to verify that authorization decisions match policy and to remediate deviations, the control limits the persistence of incorrect or missing authorization checks.

blocks

Requiring authorization processes and privilege assignments to be specified during project initiation prevents downstream components from receiving overly broad or incorrect rights that would allow unauthorized actions.

blocks

A.8.3 directly mandates and configures the authorization mechanisms (identity/group rights, dynamic access rules, isolation, credential enforcement) that stop CWE-285 exploitation; the bounded remainder is non-authorization exposure vectors such as error messages or observable discrepancies.

Hardening callouts derived

DISA STIG baselines are authored against specific weaknesses. Where this CVE cites a broad weakness class, the mapping is not asserted at that abstraction (owner ruling, 2026-09-25: grade only at Base and below), so no rule is shown here rather than a rule shown against the wrong grain.

  • No fine-grained STIG row for CWE-285, a class-level weakness. Rules are authored against its narrower children: CWE-552.

References