---
id: "CVE-2021-28799"
kind: "vulnerability"
aliases: ["CVE-2021-28799"]
title: "QNAP Network Attached Storage (NAS): QNAP NAS Improper Authorization Vulnerability"
severity: "critical"
cvss: 10.0
epss: 0.7825
exploited: true
status: "open"
category: "infrastructure"
products: [{"name": "QNAP Network Attached Storage (NAS)", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["exploited", "hbs-3", "infrastructure", "nas", "qnap", "ransomware", "self-hosted"]
not_affected_if: ["no-hbs-3-installed"]
published_at: "2021-05-13T03:15:06+00:00"
first_seen_at: "2026-10-23T19:11:52+00:00"
updated_at: "2026-10-23T21:57:35+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-28799", "first_seen_at": "2026-10-23T19:11:52+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2021-28799", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2021-28799", "first_seen_at": "2026-10-23T21:57:35+00:00"}]
---

# QNAP Network Attached Storage (NAS): QNAP NAS Improper Authorization Vulnerability

QNAP NAS devices running HBS 3 have an improper authorization flaw that lets remote attackers log in to the device. It is listed in CISA's known-exploited catalog and tagged for ransomware use.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:QNAP Network Attached Storage (NAS) affected ? fixed ?

## How to check

Check whether a QNAP NAS runs the HBS 3 (Hybrid Backup Sync) app, which the sources name as affected; the sources do not give a command or version range.

## Fix

Apply the QNAP update per vendor instructions, as CISA requires; the fixed version is unknown from these sources.
Update HBS 3 and the NAS firmware, then review login logs.

## Timeline

- 2026-10-23T19:11:52+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2021-28799
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2021-28799
- 2026-10-23T21:57:35+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2021-28799

## From the sources

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 .

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_