CVE-2021-28799
Confirmed PUBLISHEDImproper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)
Recommended Action
Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
At a Glance
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 .
- CVE Published
- May 13, 2021
- Exploitation Reported
- Mar 31, 2022
- CVSS
- 10.0 Critical
- EPSS
- 78.3%
Affected Versions
| Vendor | Product | Version | Status |
|---|---|---|---|
| QNAP Systems Inc. |
HBS 3
|
unspecified to < v16.0.0415 |
Affected |
| QNAP Systems Inc. |
HBS 3
|
unspecified to < v3.0.210412 |
Affected |
| QNAP Systems Inc. |
HBS 3
|
unspecified to < v3.0.210411 |
Affected |
| QNAP Systems Inc. |
HBS 3
|
unspecified to < v3.0.210411 |
Affected |
| QNAP Systems Inc. |
HBS 3
|
unspecified to < v16.0.0419 |
Affected |
| QNAP Systems Inc. |
HBS 3
|
unspecified to < v16.0.0419 |
Affected |
| QNAP Systems Inc. |
HBS 2
|
all versions |
Unaffected |
| QNAP Systems Inc. |
HBS 1.3
|
all versions |
Unaffected |
CVE References
- qnap.com/en/security-advisory/QSA-21-13 qnap.com · CVE Record https://www.qnap.com/en/security-advisory/QSA-21-13
Recommended Actions
- Prioritize remediation. Validate affected assets and apply vendor fixes on an accelerated timeline.
- Check enrichment artifacts for scanner coverage and available PoCs before rolling remediation validation.
- Use the Pro API to automate enrichment, telemetry, and workflow delivery for VM, SOC, and CTI pipelines.
Known Exploited Vulnerability Sources
Catalogues that list this CVE as a known exploited vulnerability.
Per-source evidence links for KEV attestations are available through the KEVIntel Pro API.
Learn about Pro API access| Source | Added |
|---|---|
| CISA First | 2022-03-31 00:00 UTC |
| The Shadowserver | 2026-07-08 00:00 UTC |
Scanner Artifacts
Nuclei and Metasploit references linked to this CVE.
| Scanner | Reference | Detected |
|---|---|---|
| Nuclei | https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-28799.yaml | Jun 01, 2026 |
Virtual Patch
Compensating WAF rules to help reduce exposure to this CVE. Rule content and deployable vendor exports are available with KEVIntel Enterprise.
KEVIntel does not currently have a virtual patch for this CVE. When available, KEVIntel virtual patches ship as deployable ModSecurity, Cloudflare, and AWS WAF rules.
Enterprise feature. Virtual patch rule content and deployable vendor exports (ModSecurity, Cloudflare, AWS WAF) are available to KEVIntel Enterprise users.
CVSS Scores
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
AV:N/AC:L/Au:N/C:P/I:P/A:P
Exploitation Status
Exploited in the wild
Recorded 2022-03-31 00:00:00 UTC · CISA
Used in malware
Recorded 2022-03-31 00:00:00 UTC · CISA
Proof of concept available
Recorded 2026-06-12 14:20:56 UTC · Nuclei Templates
Weaknesses (CWE)
-
Improper Authorization
Scanner Integrations
| Scanner | Reference | Detected |
|---|---|---|
| Nuclei | https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-28799.yaml | Jun 01, 2026 |
Potential Proof of Concepts
These PoCs are unverified and could contain malware. Use at your own risk.
nuclei · Created Unknown
Timeline
Key exploitation, disclosure, scanner coverage, and KEV attestation events for this CVE.
-
00:00 UTC 16 days ago00:00 UTC · 16 days ago
KEV confirmed by The Shadowserver
Exploitation attested by an external source
-
14:20 UTC about 1 month ago14:20 UTC · about 1 month ago
Public PoC available
Public proof-of-concept code published
-
15:34 UTC about 2 months ago15:34 UTC · about 2 months ago
Nuclei template available
Scanner coverage available
-
00:00 UTC over 4 years ago00:00 UTC · over 4 years ago
Added to CISA KEV
Listed in the CISA Known Exploited Vulnerabilities catalog
-
00:00 UTC over 4 years ago00:00 UTC · over 4 years ago
First public exploitation report
Exploit observed in malware
-
02:55 UTC about 5 years ago02:55 UTC · about 5 years ago
CVE published
Vulnerability disclosed publicly
-
00:00 UTC over 5 years ago00:00 UTC · over 5 years ago
CVE ID reserved
Identifier reserved by the CNA
Automate This Intelligence with the Pro API
Confidence scoring, exploit status, sensor telemetry, PoCs, scanner integrations, mentions, and tags are available programmatically for VM, SOC, and CTI workflows.
Pro API Example
GET /api/v2/pro/kevs/CVE-2021-28799
{
"cve_id": "CVE-2021-28799",
"title": "Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)",
"affected_vendor": "QNAP Systems Inc.",
"affected_product": "HBS 3, HBS 2, HBS 1.3",
"affected_versions": [
{ "vendor": "...", "product": "...", "status": "affected", "display_label": "..." }
],
"confidence": "Confirmed",
"cvss_score": 10.0,
"epss_score": 0.7825,
"exploit_status": {
"exploited_in_the_wild": true,
"active_exploitation_observed": false
},
"sensor_telemetry": { "...": "Pro API fields" },
"proof_of_concepts": [ "..." ],
"scanner_integrations": [ "..." ]
}