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CVE-2021-28799

9.8, critical Known exploited Used in ransomware

QNAP NAS Improper Authorization Vulnerability

NVD (opens in a new tab) CVE.org (opens in a new tab) CISA KEV catalogue (opens in a new tab)
CVSS
9.8, critical critical CVSS v3.1
EPSS, next 30 days
78% higher than 99.5% of scored CVEs
Exploitation
Confirmed in the wild
Published
13 May 20215 years ago

Known exploited

CISA added this to the Known Exploited Vulnerabilities Catalog on 31 Mar 2022 and links it to ransomware activity. US federal agencies had to remediate by 21 Apr 2022, a deadline that has passed.

Required action. Apply updates per vendor instructions.

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 .

Details

Vendor
QNAP
Product
HBS 3
Weakness
CWE-285 (opens in a new tab)
Status
Analyzed
Updated
17 Jun 2026
Assigner
security@qnapsecurity.com.tw

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

References 2

Advisories 1

Other references 1

The kind is read from NVD's tags where present and from the shape of the URL otherwise, so treat it as a hint.

Other QNAP CVEs

Every QNAP CVE we hold

Same vendor in what we hold, not a known link between the flaws.

  • CVE-2023-47565 QNAP VioStor NVR OS Command Injection Vulnerability KEV EPSS 73% 8.8, high
  • CVE-2022-27593 QNAP Photo Station Externally Controlled Reference Vulnerability KEV EPSS 88% 9.1, critical
  • CVE-2020-2509 QNAP Network-Attached Storage (NAS) Command Injection Vulnerability KEV EPSS 34% 9.8, critical
  • CVE-2018-19943 QNAP NAS File Station Cross-Site Scripting Vulnerability KEV EPSS 21% 5.4, medium
  • CVE-2018-19949 QNAP NAS File Station Command Injection Vulnerability KEV EPSS 29% 9.8, critical
  • CVE-2018-19953 QNAP NAS File Station Cross-Site Scripting Vulnerability KEV EPSS 29% 6.1, medium
  • CVE-2019-7192 QNAP Photo Station Improper Access Control Vulnerability KEV EPSS 88% 9.8, critical
  • CVE-2019-7193 QNAP QTS Improper Input Validation Vulnerability KEV EPSS 14% 9.8, critical

Beyond this page

Is CVE-2021-28799 in your network?

This page tells you how dangerous CVE-2021-28799 is out there. Whether it sits on one of your devices, only a scan of your own network can tell. That is what our systems do, every day, for the companies we protect.

CVE-2021-28799 is in the CISA catalogue of exploited vulnerabilities: attackers are already exploiting it.

  • EXYS7600 OCELOT

    Find it before attackers do

    Ocelot scans your network on a schedule and reports every vulnerability it finds by its CVE identifier, with its CVSS score and how many of your hosts are affected. A finding like CVE-2021-28799 comes with the devices to patch.

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  • EXYS7950 KONDRA

    Detect it if it's exploited

    Until the patch is applied, Kondra monitors your network, events and endpoints in real time. It keeps an inventory of the software installed on every endpoint, and its anomaly detection rules flag what an attacker does once inside.

    Learn more about EXYS7950 KONDRA
Check your exposure

Assembled from the NVD 2.0 API, the CISA Known Exploited Vulnerabilities Catalog and FIRST EPSS. Scores are as of our last collection on 17 Oct 2026, not the live upstream state.