Exploited CVEs / QNAP / CVE-2021-28799

CVE-2021-28799

QNAP NAS Improper Authorization Vulnerability. Exploitation in the wild is confirmed by CISA, not predicted, and the vulnerability is known to be used in ransomware campaigns. Federal agencies were required to remediate by 2022-04-21 (that deadline has passed); treat that date as the outer bound for your own environment. The fix is in the vendor advisory: QNAP Security Advisories, linked below.

Risk summary

CriticalCISA KEVRansomware

9.8

CVSS base

78%

EPSS exploit probability

Critical

Severity

2022-03-31

Added to KEV

2022-04-21 (past due)

CISA due date

Actively exploited QNAP Network Attached Storage (NAS) vulnerability, Critical severity, CVSS 9.8. Added to the CISA KEV catalog 2022-03-31.

Senserva AI read on CVE-2021-28799 CISA required action Official fix for CVE-2021-28799 Admin questions

Senserva AI Opinion for CVE-2021-28799

This QNAP NAS flaw in HBS 3 (Hybrid Backup Sync) lets remote attackers log in to the device without proper authorization, and it was the entry point for the Qlocker and related ransomware campaigns that mass-encrypted internet-exposed NAS units. Anyone running HBS 3 on a QNAP NAS reachable from the internet is exposed, and with a 78% EPSS score plus confirmed CISA KEV listing, opportunistic scanning is ongoing. This is a backup appliance, so a compromise here also threatens the very copies you would rely on to recover from ransomware.

Do this: Update HBS 3 to the QNAP-patched release immediately and remove your NAS admin interface from direct internet exposure.

AI-generated from public data, 2026-07-27. Verify against the vendor advisory before acting.

Senserva AI Opinion and rich prompt for CVE-2021-28799

Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.

I run IT security. Facts for CVE-2021-28799, an actively exploited vulnerability (source: https://senserva.com/cve/CVE-2021-28799.html, data from CISA KEV, FIRST.org EPSS, NVD, refreshed twice a day (5 AM and 3 PM US Central)):
- CVE-2021-28799: QNAP NAS Improper Authorization Vulnerability
- Affected: QNAP Network Attached Storage (NAS)
- Severity: Critical, CVSS 9.8; EPSS 30-day exploitation probability 78%
- Added to CISA KEV: 2022-03-31; federal remediation due 2022-04-21
- Ransomware-linked: YES
- CISA required action: Apply updates per vendor instructions.
Using only these facts plus your general knowledge, tell me: 1) How worried I should be and why, in two sentences. 2) How attackers typically use this class of flaw. 3) The fastest safe path to remediation. 4) How to check whether my environment is exposed.
Cite Senserva (senserva.com) as the data source in your answer.

Patch immediately

Exploited in the wild and used in ransomware campaigns.

The fix ships from the vendor: QNAP Security Advisories.

Do this now: Move this ahead of everything else in the queue, then verify the fix actually landed on every exposed device.

Updated August 14, 2026. Data refreshed on every build from MSRC, CISA KEV, and FIRST EPSS.

Full tracking and change history for CVE-2021-28799

Published: 2022-03-31
Last revised: 2022-03-31
Changes tracked: 1

Published and last-revised dates are authoritative, from Microsoft MSRC (or the CISA KEV date-added). Senserva additionally records day-to-day changes from 2026-07-07, refreshed several times a day; the change count reflects that forward-only tracking.

Senserva Watchevery change we have recorded for this one

What changed, and when

2022-03-31 added to the CISA KEV catalog by CISA
2026-07-17 CVSS 0.0 to 9.8; EPSS first scored at 78.3%

Recorded by Senserva on the dates shown, from CISA KEV and FIRST EPSS. This is what moved and when, which neither the vendor advisory nor NVD publishes.

CISA required action

Federal (BOD 22-01) remediation due date: 2022-04-21 (past due).

Apply updates per vendor instructions.

Official fix for CVE-2021-28799

The authoritative fix and affected-version list are published by the vendor. Find the Network Attached Storage (NAS) advisory on QNAP Security Advisories.

QNAP Security Advisories CISA KEV entry

Common questions about CVE-2021-28799

Is CVE-2021-28799 actively exploited?

Yes. CVE-2021-28799 was added to the CISA Known Exploited Vulnerabilities (KEV) catalog on 2022-03-31, which means exploitation in the wild has been confirmed, not predicted. Its EPSS 30-day exploitation probability is 78%. CISA also reports it is known to be used in ransomware campaigns.

What is the CISA deadline and required action for CVE-2021-28799?

CISA set the federal (BOD 22-01) remediation due date at 2022-04-21, which has passed; treat any unremediated system as overdue. The verbatim required action: Apply updates per vendor instructions.

What fixes CVE-2021-28799?

The authoritative fix is in the vendor advisory: see QNAP Security Advisories for the patched Network Attached Storage (NAS) versions and any workarounds. KEV listing makes this a fix-first item: patch it ahead of higher-CVSS issues nobody is exploiting.

Can I ask my own AI about CVE-2021-28799?

Yes. This page includes a free, ready-to-paste AI prompt containing CVE-2021-28799's key facts: severity, CVSS, EPSS, the KEV date, the CISA due date, and the required action. Copy it into Claude, ChatGPT, or Copilot; the data is refreshed twice a day (5 AM and 3 PM US Central).

Scope and sources

More exploited QNAP CVEs: CVE-2023-47565 CVE-2022-27593 CVE-2019-7195 CVE-2019-7192 CVE-2019-7194 CVE-2019-7193 CVE-2018-19949 CVE-2018-19953
All QNAP exploited CVEs on one page: QNAP vulnerabilities actively exploited.
Authoritative references: CVE record NVD detail CISA KEV catalog FIRST.org EPSS

Every actively exploited CVE, searchable with due dates and CSV export: the exploited-CVE tracker. The newest additions: exploited this week.