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critical·Product Based Web Vulnerabilities·Updated Jan 10, 2026

CVE-2021-28799 Scanner

CVE-2021-28799 Scanner - Improper Access Control vulnerability in Hybrid Backup Sync (HBS 3)

Est. Time~10 seconds
Scan TypeSingle Scan
Targetsdomain, subdomain, ipv4
CostFree
3.5k
Times Used
continuous scan runs
4.1k
Continuously Checked
assets under CS
0
Vulnerabilities Found
confirmed findings
References
🔴
CISA Known Exploited Vulnerability
This CVE is actively exploited in the wild. CISA mandates federal agencies to patch immediately.
CVECVE-2021-28799
9.8
CVSScritical
Exploitable remotely over the internet · no authentication required.

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 .

Attack Vector
Network
Privileges Req.
None
User Interaction
None
Affected
HBS 3by QNAP Systems Inc.
AFFECTED< v16.0.0415SAFE ✓≥ v16.0.0415
HBS 3by QNAP Systems Inc.
AFFECTED< v3.0.210412SAFE ✓≥ v3.0.210412
HBS 3by QNAP Systems Inc.
AFFECTED< v3.0.210411SAFE ✓≥ v3.0.210411
HBS 3by QNAP Systems Inc.
AFFECTED< v3.0.210411SAFE ✓≥ v3.0.210411
Updated Aug 21, 2026View on NVD →
Detail

Hybrid Backup Sync (HBS 3) is a comprehensive backup and disaster recovery solution developed by QNAP Systems, Inc. Primarily used within network-attached storage (NAS) systems, it's designed to provide data protection, retrieval, and synchronization solutions. Businesses and individual users utilize HBS 3 to safeguard their data by executing file, folder, and system-level backups. It supports various protocols and integrates seamlessly with cloud storage platforms, making it highly versatile. The solution is critical in environments where data integrity and availability are paramount. HBS 3 ensures that sensitive data can be securely stored and easily restored in the event of data loss or corruption.

The Improper Access Control vulnerability in HBS 3 allows remote attackers to exploit a system without appropriate authorization. This vulnerability is particularly concerning as it could enable unauthorized data access and full system compromise. Access control mechanisms are intended to protect sensitive data by ensuring only authorized users can access certain resources, and when these controls are weak or misconfigured, they can easily be bypassed. In HBS 3, this vulnerability may exist due to a lack of proper authentication mechanisms or checks in the software's code, opening up the risks for exploitation. Its presence emphasizes the need for robust security checks and processes in software dealing with data management and backup.

The vulnerability resides in the Backup Management functionality, which processes commands based on their respective session identifiers. By manipulating the session identifier "act":"run_cmd", unauthorized commands such as "cmd":"cat /etc/passwd" can potentially be executed. Attackers exploit the raw POST requests sent to the endpoint "/cgi-bin/backup/hbs_mgnt.cgi". The crafted payload would result in revealing sensitive information stored within the targeted system. Furthermore, explanations of incorrect login bypass highlight how effective remote exploitations can occur without needing authenticated access.

If exploited, this vulnerability can lead to unauthorized access to critical systems, resulting in data breaches. Potential threats include data theft, modification, or deletion, which can disrupt operations and result in financial and reputational damage. Unauthorized system access can also lay groundwork for further exploits, like installing malware, creating persistence within the system, or escalating privileges. Due to its severity, businesses could face compliance issues with data protection standards and governance.

REFERENCES

Solution Advice
  • Update to the latest versions: v16.0.0415 or later for QTS 4.5.2, v3.0.210412 or later for QTS 4.3.6, v3.0.210411 or later for QTS 4.3.4 and 4.3.3, v16.0.0419 or later for QuTS hero h4.5.1, and v16.0.0419 or later for QuTScloud c4.5.1~c4.5.4.
  • Implement network segmentation to minimize access to the NAS devices.
  • Implement a robust monitoring and alerting solution to detect unauthorized access attempts in real-time.
  • Enforce strong passwords and multifactor authentication for all administrative accesses.
  • Conduct regular security audits and tests to identify and mitigate vulnerabilities promptly.

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