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CVE-2021-28809

38
FAUCET Score

CVE-2021-28809 is an improper access control vulnerability affecting legacy versions of QNAP HBS 3 and QTS, allowing attackers to compromise the operating system. With a CVSS score of 9.8 (Critical), this vulnerability is easily exploitable over the network without authentication, leading to complete compromise of confidentiality, integrity, and availability. While not listed on CISA's KEV catalog, it has garnered some community discussion and media coverage, though no public exploit code (Metasploit, Nuclei, ExploitDB) is currently available.

Impacted Technologies

VendorProductVersion(s)CPE
< 3.0.210507CPE matchmatch criteria
cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
< 3.0.210506CPE matchmatch criteria
cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

9.8CRITICAL

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
Exploitability Score
3.9
Impact Score
5.9
CvssVersion
3.1

Exploit Intelligence

EPSS Score
15.80%
Probability of exploitation in next 30 days
EPSS Percentile
96.5%
Percentile rank of EPSS score among Peer Group
As of 2026-07-28
Model: v2026.06.15
This CVE's current EPSS score of 0.1580 is in the 93rd percentile among its peer group of 36,897 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.1 Reddit, 0.3 Bluesky, 0.3 Mastodon, and 2.4 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.3 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Remediation records are not available for this CVE.

References

qnap.com / en/security-advisory/qsa-21-19
Vendor Advisory
zerodayinitiative.com / advisories/ZDI-21-783
Third Party AdvisoryVDB Entry