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365 AppsApplication · Microsoft

CVE-2026-21258

MEDIUM · 5.5 CVSS v3.1 Published 2026-02-10
Fix available
A fix is available. Upgrade to 16.0.10417.20097 or later.
See remediation →
58/100
Remediation priority · Elevated
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
Improper input validation in Microsoft Office Excel allows an unauthorized attacker to disclose information 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

This is an improper input validation vulnerability in Microsoft Office Excel that allows a local unauthorized attacker to potentially disclose sensitive information. The vulnerability stems from insufficient validation of input data within Excel, which could be exploited to access information beyond the user's intended permissions.

MitigationOrganizations should apply Microsoft security updates for Office Excel as they become available, and ensure employees are running current, patched versions of Microsoft Office to mitigate this local information disclosure risk.

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
None
Availability
None

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

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

  1. Check Microsoft Excel desktop application version
    Open Excel, go to File > Account > About Excel. Alternatively, run 'winword.exe /?' or 'excel.exe /?' from command prompt, or check Add/Remove Programs for Microsoft Office installation.
    Affected if The installed Excel version is 2016 or later, or the Office version is 2019 or later, meaning the application is in the affected version range
  2. Check Microsoft 365 Apps version
    Open any Microsoft 365 application, go to File > Account. Look for 'Update Options' or check via Office Admin Center for deployed versions.
    Affected if Microsoft 365 Apps are installed - all versions are affected according to the advisory
  3. Check Microsoft Office Long Term Servicing Channel version
    In Office deployment tools or Office Admin Center, check the LTSC version installed. Run 'ospp.vbs /dstatus' or check installed programs for Microsoft Office LTSC 2021 or 2024.
    Affected if Microsoft Office LTSC 2021 or 2024 is installed - both versions are in the affected range
  4. Check Microsoft Office Online Server build version
    On the Office Online Server, open SharePoint Management Shell and run: Get-OfficeOnlineServerHealth. Or check the OfficeOnlineServer build number via Windows Registry at HKLM\SOFTWARE\Microsoft\Office Web Apps\Server\Config.
    Affected if Office Online Server build is lower than 16.0.10417.20097 (e.g., 16.0.10417.xxxx or earlier)

The environment is affected if any version of Microsoft Excel (2016+), Microsoft Office (2019+), Microsoft 365 Apps, or Office Online Server (builds below 16.0.10417.20097) is installed, as the input validation flaw exists in all these vulnerable versions.

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

Organizations should apply Microsoft security updates for Office Excel as they become available, and ensure employees are running current, patched versions of Microsoft Office to mitigate this local information disclosure risk.

Recommended fix High confidence

Microsoft 365 Apps: install latest updates; Office Online Server: version 16.0.10417.20097 or later

  1. 1. Check current Microsoft Office/Excel version by opening Excel, going to File > Account > About Excel
  2. 2. For Microsoft 365 Apps, Office 2016, Office 2019, Office 2021, and Office 2024: Run Microsoft Update or open any Office app > File > Account > Update Options > Update Now to receive the latest security patches
  3. 3. For Office Online Server: Update to version 16.0.10417.20097 or later by installing the latest Office Online Server cumulative update from the Microsoft Update Catalog
  4. 4. Verify the update was applied by checking the version number again after rebooting any servers
Caveat Standard Office update risks - ensure compatibility testing for mission-critical templates before deploying organization-wide

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

Fix this in 365 Apps Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,696.

Scan for this in your stack

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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-2026-21258 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