CVE-2021-28799 — Hybrid Backup Sync
CRITICALCVE-2021-28799 is a critical-severity (CVSS 10) CWE-285 vulnerability in qnap hybrid backup sync. It is in CISA's Known Exploited Vulnerabilities catalog (added 2022-03-31) and has been used in ransomware campaigns — treat it as actively exploited and patch now. A fix is available — see the affected versions and patch details below.
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…
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2022-03-31 — including use in ransomware campaigns. Federal agencies were required to remediate by 2022-04-21.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-28799.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-28799 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 395,308 CVEs with a current EPSS score, this one falls in the 50–90% band (highlighted). Counts from FIRST.org, log-scaled.
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 .
Affected Products
hybrid backup syncqnapResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Vendor / applianceDetect
Inventory every qnap hybrid backup sync deployment and check each version against the affected-products list above.
Fix
Apply the qnap hybrid backup sync security patch or hotfix for CVE-2021-28799 on the affected version, following the vendor advisory for your exact build.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
How to detect CVE-2021-28799
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2021-28799 -u https://target- Template
- QNAP HBS 3 - Broken Access Control
- Severity
- critical
- Impact
- Remote attackers can log in without proper authorization, potentially leading to full system compromise or unauthorized data access.
- Remediation
- 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.
Template by ProjectDiscovery nuclei-templates (daffainfo), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2021-28799 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.