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CVE-2021-28799

96
FAUCET Score

CVE-2021-28799 is a critical improper authorization vulnerability in QNAP NAS devices running specific versions of HBS 3 (Hybrid Backup Sync), allowing remote attackers to log in. With a CVSS score of 9.8, it presents a critical risk due to its network-based attack vector, low complexity, and complete compromise of confidentiality, integrity, and availability. This vulnerability is actively exploited, notably in ransomware campaigns, and has garnered significant community discussion and media coverage, despite the absence of public Metasploit or ExploitDB modules.

Impacted Technologies

VendorProductVersion(s)CPE
< 16.0.0415CPE matchmatch criteria
cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
< 3.0.210412CPE matchmatch criteria
cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
< 3.0.210411CPE matchmatch criteria
cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*
< 16.0.0419CPE matchmatch criteria
cpe:2.3:a:qnap:hybrid_backup_sync:*:*:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

10.0CRITICAL

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

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
CHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
Exploitability Score
3.9
Impact Score
6.0
CvssVersion
3.1

Exploit Intelligence

EPSS Score
78.25%
Probability of exploitation in next 30 days
EPSS Percentile
99.5%
Percentile rank of EPSS score among Peer Group
As of 2026-07-27
Model: v2026.06.15
Added to KEV · Mar 31, 2022
Nuclei: CVE-2021-28799 · Jan 9, 2026
This CVE's current EPSS score of 0.7825 is in the 98th 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

cisa.gov / known-exploited-vulnerabilities-catalog
US Government Resource
qnap.com / en/security-advisory/QSA-21-13
Vendor Advisory