Security readout for executives and security teams
Plain-English summary
This flaw lets a remote attacker log in to affected QNAP NAS devices running vulnerable HBS 3. Because HBS handles backup and sync operations, unauthorized access can threaten stored data, backup integrity, and availability. The CVSS score is 10, and CISA lists it as known exploited.
Executive priority
Treat this as urgent for any QNAP NAS running HBS 3. Known exploitation, unauthenticated remote access, and backup-system impact create a high business risk. Patch affected systems promptly and verify no exposed device remains below the fixed version.
Technical view
CVE-2021-28799 is an improper authorization issue in QNAP HBS 3. It affects listed HBS 3 versions on QTS 4.5.2, 4.3.6, 4.3.4, 4.3.3, QuTS hero h4.5.1, and QuTScloud c4.5.1 through c4.5.4 before fixed builds. HBS 2 and HBS 1.3 are listed as unaffected.
Likely exposure
Exposure is likely where QNAP NAS devices run HBS 3 below the fixed version for their QTS, QuTS hero, or QuTScloud platform. Internet-reachable NAS systems should be treated as highest priority because the vulnerability is network-accessible and needs no prior authentication.
Exploitation context
CISA KEV indicates known exploitation. The provided sources do not describe exploit mechanics, campaign details, indicators of compromise, or attacker tooling. The advisory impact is remote login to the device, which can materially affect confidentiality, integrity, and availability.
Researcher notes
The source bundle confirms product, version thresholds, CVSS vector, and KEV status, but not exploit procedure or observed campaigns. Validation should focus on version evidence, device exposure, and post-remediation confirmation rather than exploit reproduction.
Mitigation direction
- Upgrade HBS 3 to the fixed version matching the device platform listed by QNAP.
- For QTS 4.5.2, update HBS 3 to v16.0.0415 or later.
- For QTS 4.3.6, update HBS 3 to v3.0.210412 or later.
- For QTS 4.3.4 or 4.3.3, update HBS 3 to v3.0.210411 or later.
- For QuTS hero or QuTScloud listed versions, update HBS 3 to v16.0.0419 or later.
- If patching is blocked, check QNAP guidance for supported temporary mitigations.
Validation and detection
- Inventory all QNAP NAS devices and record OS platform and HBS version.
- Confirm whether each device runs HBS 3, HBS 2, or HBS 1.3.
- Compare HBS 3 versions against the QNAP fixed-version matrix.
- Prioritize validation for devices reachable from untrusted networks.
- Review authentication logs for unexpected successful logins where logging is available.
- Track remediation status separately for each QTS, QuTS hero, and QuTScloud device.
Public sources used
Michael Williams reviewed this cited source version on .
Conservative CVE-to-ATT&CK context
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Open ATT&CK lookupCVE-2021-28799 mapping review
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Open ATT&CK lookup- Severity
- Critical
- CVSS
- 10 (3.1)
- Known Exploited
- Yes
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H3.96Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
10CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://www.qnap.com/en/security-advisory/QSA-21-13CVE reference · x_refsource_MISC
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-28799CVE reference · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Authorization
Improper Authorization represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.