Cyber 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 99.5th percentile
Risk Priority 100 floored blend · peak EPSS

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, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 0.5% 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

MITRE ATT&CK Enterprise Techniques

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
T1548 Abuse Elevation Control Mechanism Privilege Escalation
Adversaries may circumvent mechanisms designed to control privilege elevation to gain higher-level permissions.
T1548.001 Setuid and Setgid Privilege Escalation
An adversary may abuse configurations where an application has the setuid or setgid bits set in order to get code running in a different (and possibly more privileged) user’s context.
T1548.002 Bypass User Account Control Privilege Escalation
Adversaries may bypass UAC mechanisms to elevate process privileges on system.
T1548.003 Sudo and Sudo Caching Privilege Escalation
Adversaries may perform sudo caching and/or use the sudoers file to elevate privileges.
T1574.005 Executable Installer File Permissions Weakness Stealth
Adversaries may execute their own malicious payloads by hijacking the binaries used by an installer.
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2023-50363Same product: Qnap Qts
CVE-2023-32967Same product: Qnap Qts
CVE-2020-2509Same product: Qnap Qtsboth on KEV
CVE-2019-7192Same product: Qnap Qtsboth on KEV
CVE-2024-21905Same product: Qnap Qts
CVE-2023-45035Same product: Qnap Qts
CVE-2023-41275Same product: Qnap Qts
CVE-2023-41277Same product: Qnap Qts
CVE-2023-41292Same product: Qnap Qts
CVE-2023-41279Same product: Qnap Qts

Affected Assets

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

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

Detect
Catch it (NIST detect / respond)

Harden
Shrink the surface (DISA STIG)
  • 6 hardening rules · 3 OS baselines
Validate
Prove the fix (OWASP ASVS)

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 (NIST CSF 2.0) AI

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

PR.AA-05 mostly match
prevents

PR.AA-05 mostly prevents CWE-285 via enforced policy, reviews, and least-privilege authorization decisions, yet CWE-285 remains only partially prevented because code-level check omissions or errors can still occur outside that single control.

PR.PS-06 mostly match
prevents

Secure SDLC practices catch and eliminate most authorization defects before release, yet a single broad outcome cannot address every design, role, and runtime facet of CWE-285.

PR.AA-01 partial match
prevents

PR.AA-01 supplies managed identities and access-request workflows that can support downstream authorization decisions, yet does nothing to enforce or verify authorization checks inside a product, leaving CWE-285 fully unaddressed by this control alone.

PR.IR-01 partial match
prevents

Network segmentation/zero-trust limits external reachability (partial prevention of exploitation) but leaves application-level authorization logic untouched, so the CWE remains fully introducible and only one facet of its risk is addressed.

PR.PS-05 none match
prevents

PR.PS-05 blocks unauthorized binaries/DNS while CWE-285 is an in-product authorization-check defect, so the control neither prevents the weakness nor removes any of its risk.

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.

prevents

Granular, policy-driven assignment of permissions and dynamic enforcement of those permissions prevent the incorrect or missing authorization decisions that lead to improper authorization flaws.

prevents

By requiring documented authorization rules and periodic policy reviews, the control makes it less likely that authorization decisions will be omitted or implemented inconsistently across applications.

prevents

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.

prevents

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.

Hardening callouts derived

Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).

Oracle Linux 8 (2 rules)
  • V-252656 The OL 8 operating system must not be configured to bypass password requirements for privilege escalation. prevents CWE-285
  • V-248581 OL 8 must require users to provide a password for privilege escalation. prevents CWE-285
RHEL 7 (3 rules)
  • V-204429 The Red Hat Enterprise Linux operating system must be configured so that users must provide a password for privilege escalation. prevents CWE-285
  • V-204430 The Red Hat Enterprise Linux operating system must be configured so that users must re-authenticate for privilege escalation. prevents CWE-285
  • V-251704 The Red Hat Enterprise Linux operating system must not be configured to bypass password requirements for privilege escalation. prevents CWE-285
RHEL 8 (1 rule)
  • V-251712 The RHEL 8 operating system must not be configured to bypass password requirements for privilege escalation. prevents CWE-285

References