CVE-2026-21509
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 · uneditedReliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
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 confidenceMicrosoft Office contains a vulnerability where the application relies on untrusted inputs when making security decisions, allowing a local unauthenticated attacker to bypass a security feature. This likely involves improper validation of input data that should be trusted for security-critical operations.
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 dataall versions= 2016= 2019= 2021= 2024CVSS 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
- Local
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/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 checksWork through these to decide whether this CVE applies to you.
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Identify installed Office versionOpen any Office application (e.g., Word, Excel), go to File > Account > About [App], or check registry key HKLM\SOFTWARE\Microsoft\Office\ClickToRun\Configuration for Microsoft 365 Apps, or HKLM\SOFTWARE\Microsoft\Office\16.0\Common\InstallRoot for Office 2016/2019/2021Affected if The installed version matches Microsoft 365 Apps (any version), Office 2016, Office 2019, Office LTSC 2021, or Office LTSC 2024
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Determine if local security boundaries are relevantReview the execution environment for Office: check if the device is shared or has multiple local users, as the vulnerability allows a local attacker to bypass a security feature via untrusted inputsAffected if Multiple local users exist or the Office environment has limited isolation between users and documents
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Assess document protection mechanismsExamine Office Trust Center settings (File > Options > Trust Center > Trust Center Settings) for any security features that process external or untrusted content, such as file validation, macro settings, or ActiveX controlsAffected if Security features that rely on input validation are enabled and could accept untrusted local inputs
A user is affected if their Office version falls within the listed affected versions (Microsoft 365 Apps all versions, Office 2016/2019, Office LTSC 2021/2024) and the environment allows local attacker access to influence security decisions.
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 dataApply Microsoft Office security updates as they become available to address the input validation vulnerability enabling security feature bypass.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,224.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-21509 — 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-2026-21509 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
- No spam, self-promotion, credentials, or personal data