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Back to active CVEs
Critical · CVSS 10.0Known ExploitedCVE-2021-28799

Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync)CVE-2021-28799

Answer in brief

CVE-2021-28799 records a Critical severity (CVSS 10.0) vulnerability in Improper Authorization Vulnerability in HBS 3 (Hybrid Backup Sync). The current sources mark it as known exploited. The current feed maps QNAP Systems Inc./HBS 3 (generic), QNAP Systems Inc./HBS 3 (generic), QNAP Systems Inc./HBS 3 (generic), QNAP Systems Inc./HBS 3 (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published May 13, 2021Updated Oct 21, 2025Source checked Nov 3, 2026First seen by HOL May 24, 2026Material review Oct 21, 2025
Upstream Advisory

Key facts

Risk
Critical · CVSS 10.0
Exploitation
Known exploited
Affected software
4 mapped packages or products
Fix availability
Available

Why this deserves its current priority

CVSS is 10.0. Known-exploitation status makes exposure review time-sensitive. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps QNAP Systems Inc./HBS 3 (generic), QNAP Systems Inc./HBS 3 (generic), QNAP Systems Inc./HBS 3 (generic), QNAP Systems Inc./HBS 3 (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
QNAP Systems Inc./HBS 3generic>=unspecified <v16.0.0415v16.0.0415
QNAP Systems Inc./HBS 3generic>=unspecified <v3.0.210412v3.0.210412
QNAP Systems Inc./HBS 3generic>=unspecified <v3.0.210411v3.0.210411
QNAP Systems Inc./HBS 3generic>=unspecified <v16.0.0419v16.0.0419

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for QNAP Systems Inc./HBS 3, QNAP Systems Inc./HBS 3, QNAP Systems Inc./HBS 3.
  2. 2Review the reported fix. Source-reported solution: QNAP have already fixed this vulnerability in the following versions of HBS 3: QTS 4.5.2: HBS 3 v16.0.0415 and later QTS 4.3.6: HBS 3 v3.0.210412 and later QTS 4.3.3 and 4.3.4: HBS 3 v3.0.210411 and later QuTS hero h4.5.1: HBS 3 v16.0.0419 and later QuTScloud c4.5.1~c4.5.4: HBS 3 v16.0.0419 and later

Evidence timeline and material changes

  1. Published upstream

    May 13, 2021

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Oct 21, 2025

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    May 24, 2026

  4. Added to CISA KEV

    Mar 31, 2022

    Evidence: source:kev:kev:kev:record

Sources and claim methodology

  • CISA Known Exploited Vulnerabilities catalogcisa.gov
  • Source referenceqnap.com
Upstream source 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 .

Quoted source text, attributed separately from HOL analysis.

Record context

Vulnerability class
Privilege Escalation
EPSS
Not reported
CWE IDs
CWE-285
Source
CVE List V5
Source checked
Nov 3, 2026
References
2 linked sources
Open source record