Atomicorp WAF Research Note: CVE-2021-28799

QNAP HBS 3 - Broken Access Control

Research Notes

This page documents research related to exploit techniques, attack behaviors, testing results, and WAF rule interactions associated with this vulnerability.

Research finding: this note documents observed WAF protections relevant to exploit techniques associated with this vulnerability.

These notes are not intended to represent the full set of vulnerabilities mitigated by Atomicorp products and should not be interpreted as a coverage matrix, certification list, or census of protected CVEs.

Atomicorp WAF protections are primarily designed around attack techniques and exploit behaviors rather than individual vulnerability identifiers. A published research note documents a positive research finding related to that CVE. If a CVE is absent from these notes, no conclusion should be drawn about protection status.

Vulnerability 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 .

Public Vulnerability References

• NVD
• CVE Record

CVE Details

• CVE: CVE-2021-28799
• Name: QNAP HBS 3 - Broken Access Control
• Severity: CRITICAL
• CVSS: 9.8 (Critical)
• CVSS source: N/A
• Vendor: qnap
• Product: hybrid_backup_sync
• Latest protected observation: 2026-07-22
• Rule IDs observed: 344360

Test or Research Source

• Source: http/cves/2021/CVE-2021-28799.yaml
• Tags: cve, cve2021, hbs3, kev, qnap, qts, qutshero, rce, vkev

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

• Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
• Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
• Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
• Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
• Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ .