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365 AppsApplication · Microsoft

CVE-2026-21259

HIGH · 7.8 CVSS v3.1 Published 2026-02-10
Fix available
A fix is available. Upgrade to 16.0.10417.20097 or later.
See remediation →
81/100
Remediation priority · High
No privileges

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
Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to elevate privileges 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 confidence

Heap-based buffer overflow vulnerability in Microsoft Office Excel that allows local privilege elevation. The vulnerability involves memory corruption in Excel's processing of specially crafted files, enabling a local attacker to execute code with elevated privileges.

MitigationApply Microsoft security updates for Office Excel as they become available. Prioritize patching endpoint systems, particularly those with local user access or shared workstation environments where untrusted files may be processed.

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
365 AppsApplication
Affected:all versions
ExcelApplication
Affected:= 2016
OfficeApplication
Affected:= 2019
Office Long Term Servicing ChannelApplication
Affected:= 2021= 2024
Office Online ServerApplication
Affected:< 16.0.10417.20097

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
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 checks

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

  1. Verify Excel is installed
    Open Excel, go to File > Account, or run 'winword --version' or 'excel --version' via command prompt
    Affected if Excel application is present on the system
  2. Determine Excel version number
    In Excel, go to File > Account > About Excel, or check the installed programs list in Windows Settings, or use: Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Office\ClickToRun\Configuration' | Select-Object PlatformVersion,VersionToReport
    Affected if Version matches 2016, 2019, 2021, 2024, or is a Microsoft 365 Apps version (all versions affected)
  3. Check Office Online Server version (if applicable)
    For Office Online Server, run: Get-OfficeOnlineServerVersion or check the installed version via Programs and Features
    Affected if Office Online Server version is lower than 16.0.10417.20097
  4. Confirm user processes untrusted Excel files
    Review whether the system routinely opens Excel files from untrusted sources, email attachments, or shared network locations
    Affected if Users on the system open Excel files from sources outside the organization or without virus scanning

If Excel or Office is installed and the version falls within 2016, 2019, 2021, 2024, Microsoft 365 Apps, or Office Online Server below 16.0.10417.20097, and users process Excel files, the system may be affected by this vulnerability.

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 16.0.10417.20097 or later
Fixed in 16.0.10417.20097
Interim mitigation

Apply Microsoft security updates for Office Excel as they become available. Prioritize patching endpoint systems, particularly those with local user access or shared workstation environments where untrusted files may be processed.

Recommended fix High confidence

Microsoft Office security update (latest available via Windows Update/MSUpdate)

  1. Open Microsoft Word (or any Office app), go to File > Account > Update Options > Update Now to receive the latest patches
  2. Alternatively, open Windows Update (Settings > Update & Security > Windows Update) and check for updates
  3. For enterprise deployments, deploy the security update via Windows Server Update Services (WSUS) or Microsoft Configuration Manager
  4. For Office Online Server, install version 16.0.10417.20097 or later from the Microsoft Update Catalog

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

Fix this in 365 Apps Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,176.

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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-2026-21259 in production — separate from our analysis above.

No notes yet

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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