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Hybrid Backup SyncApplication · Qnap

CVE-2021-28809

CRITICAL · 9.8 CVSS v3.1 Published 2021-07-08
Fix available
A fix is available. Upgrade to 3.0.210506 / 3.0.210507 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
An improper access control vulnerability has been reported to affect certain legacy versions of HBS 3. If exploited, this vulnerability allows attackers to compromise the security of the operating system.QNAP have already fixed this vulnerability in the following versions of HBS 3: QTS 4.3.6: HBS 3 v3.0.210507 and later QTS 4.3.4: HBS 3 v3.0.210506 and later QTS 4.3.3: HBS 3 v3.0.210506 and later

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

This is an improper access control vulnerability in QNAP's HBS 3 (Hybrid Backup Sync) application affecting certain legacy versions. When exploited, it allows attackers to compromise the security of the operating system, likely through bypassing access controls to gain elevated privileges or execute unauthorized operations.

MitigationUpdate HBS 3 to the fixed versions (v3.0.210507 or later for QTS 4.3.6, v3.0.210506 or later for QTS 4.3.4/4.3.3) as specified in the advisory to remediate this vulnerability.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Hybrid Backup SyncApplication
Affected:< 3.0.210507< 3.0.210506

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm HBS 3 is installed
    Log into QTS as admin, open App Center, and search for 'Hybrid Backup Sync' in the installed applications list. Alternatively, run 'getcfg -f /etc/config/qpkg.conf -d' to list installed QNAP packages and look for 'Hybrid Backup Sync' entry.
    Affected if HBS 3 is not installed on the system (not affected in this case)
  2. Locate the HBS 3 version
    In QTS, go to App Center, find Hybrid Backup Sync, and note the version number displayed. Alternatively, check via SSH by running 'qpkg -d | grep -i hybrid' which will show package details including version.
    Affected if Version cannot be determined (insufficient data to confirm vulnerability status)
  3. Compare installed version against affected ranges
    Compare the found version to the vulnerable ranges: versions earlier than 3.0.210507 (for QTS 4.3.6 systems) and versions earlier than 3.0.210506 (for QTS 4.3.4/4.3.3 systems). Note that version 3.0.210507 and 3.0.210506 and later are fixed.
    Affected if Installed version is below 3.0.210506 or below 3.0.210507 depending on QTS version (indicates vulnerability present)
  4. Identify QTS version for version-specific assessment
    In QTS, go to Control Panel > System > System Status > System Information to find the QTS version (e.g., 4.3.6, 4.3.4, 4.3.3). Alternatively, run 'getcfg -f /etc/default_config/uLinux.conf -d System_Version' via SSH.
    Affected if QTS version is 4.3.3, 4.3.4, or 4.3.6 and HBS 3 version is below the corresponding fixed version

The environment is affected if QNAP HBS 3 is installed with a version number lower than 3.0.210506 or 3.0.210506 depending on the QTS version in use.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 3.0.210506 / 3.0.210507 or later
Fixed in 3.0.2105063.0.210507
Interim mitigation

Update HBS 3 to the fixed versions (v3.0.210507 or later for QTS 4.3.6, v3.0.210506 or later for QTS 4.3.4/4.3.3) as specified in the advisory to remediate this vulnerability.

Recommended fix High confidence

HBS 3 v3.0.210507 (QTS 4.3.6) or HBS 3 v3.0.210506 (QTS 4.3.4/4.3.3)

  1. 1. Identify the current QTS version running on the QNAP NAS (accessible via Control Panel > System > General Settings > System Version)
  2. 2. If running QTS 4.3.6, upgrade HBS 3 to version 3.0.210507 or later
  3. 3. If running QTS 4.3.4 or QTS 4.3.3, upgrade HBS 3 to version 3.0.210506 or later
  4. 4. Upgrade HBS 3 through the QNAP App Center or by installing the latest version from the QNAP website
  5. 5. Alternatively, update the entire QTS firmware which will include the updated HBS 3 package
  6. 6. After upgrade, verify the HBS 3 version reflects the fixed release (3.0.210507 or 3.0.210506)

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Hybrid Backup Sync Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing3.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,210
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-28809 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data