CVE-2021-28806
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 · uneditedA DOM-based XSS vulnerability has been reported to affect QNAP NAS running QTS and QuTS hero. If exploited, this vulnerability allows attackers to inject malicious code. This issue affects: QNAP Systems Inc. QTS versions prior to 4.5.3.1652 Build 20210428. QNAP Systems Inc. QuTS hero versions prior to h4.5.2.1638 Build 20210414. QNAP Systems Inc. QuTScloud versions prior to c4.5.5.1656 Build 20210503. This issue does not affect: QNAP Systems Inc. QTS 4.3.6; 4.3.3.
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 · high confidenceA DOM-based XSS vulnerability in QNAP NAS web interfaces (QTS, QuTS hero, QuTScloud) allows attackers to inject malicious JavaScript code through unsanitized user input that gets processed client-side and inserted into the DOM without proper validation.
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< 4.5.3.1652< h4.5.2.1638< c4.5.5.1656CVSS 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
- Low
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
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 checksWork through these to decide whether this CVE applies to you.
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Identify QNAP product typeLog into the QNAP admin web interface and check the dashboard or System Settings > General Settings to confirm whether the device runs QTS, QuTS hero, or QuTScloud.Affected if The product is QTS, QuTS hero, or QuTScloud.
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Locate the firmware versionIn the web interface, go to Control Panel > System Settings > Firmware Update, or check the System Dashboard for the current firmware version number. Alternatively, access the device via SSH and run: `cat /etc/version`Affected if The version number is displayed.
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Compare version to affected rangesCompare the installed version against these thresholds: QTS < 4.5.3.1652, QuTS hero < h4.5.2.1638, QuTScloud < c4.5.5.1656. Note that version formats may vary (numeric for QTS, prefixed with 'h' for QuTS hero, 'c' for QuTScloud).Affected if The installed version is lower than the corresponding threshold for your product.
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Verify web interface is accessibleConfirm that the QNAP web administration interface (typically on port 808 or 443) is reachable, as this is where the DOM-based XSS would be triggered.Affected if The web interface is exposed and accessible to the attacker.
You are affected if your QNAP device runs QTS, QuTS hero, or QuTScloud with a version below the respective threshold (4.5.3.1652, h4.5.2.1638, or c4.5.5.1656) and the web interface is accessible.
Generated from the published advisory. Verify against your own configuration.
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.
From vendor data4.5.3.1652
Update affected QNAP devices to QTS 4.5.3.1652+, QuTS hero h4.5.2.1638+, or QuTScloud c4.5.5.1656+ to remediate the vulnerability.
- Consultation2.0 h
- Implementation2.0 h
- Testing1.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-28806 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-28806 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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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.
- 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
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