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365 AppsApplication · Microsoft

CVE-2026-20957

HIGH · 7.8 CVSS v3.1 Published 2026-01-13
Fix available
A fix is available. Upgrade to 16.0.10417.20083 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
Integer underflow (wrap or wraparound) in Microsoft Office Excel allows an unauthorized attacker to execute code 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 integer underflow (wrap or wraparound) vulnerability in Microsoft Office Excel that allows an unauthorized local attacker to execute arbitrary code. The integer underflow likely occurs during parsing or processing of Excel file data, leading to memory corruption that can be leveraged for code execution.

MitigationApply the appropriate Microsoft security updates for Microsoft Office Excel as soon as they become available. Until patches are available, exercise caution when opening Excel files from untrusted sources.

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:< 16.0.10417.20083

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 installation on Windows
    Open Control Panel > Programs and Features, or run 'Get-ItemProperty HKLM:\Software\Microsoft\Windows\CurrentVersion\Uninstall\* | Select-Object DisplayName, DisplayVersion' in PowerShell to list installed Microsoft Office products
    Affected if Microsoft Excel is not listed among installed programs means the system is not affected by this specific Excel vulnerability
  2. Determine installed Excel desktop version
    Open Excel, go to File > Account > About Excel, or run 'powershell -Command "(New-Object -ComObject Excel.Application).Version"' to retrieve the version number
    Affected if The version falls within these affected ranges: Excel 2016 and later, Office 2019 and later, Office LTSC 2021 and later, or Office LTSC 2024 and later, OR the version is newer than current with no patch available
  3. Check Microsoft 365 Excel version
    Open Excel, go to File > Account, and look under 'Update Options' to see the version, or check via Office Apps: File > Account > About (the version format will be like 16.0.xxxxx.xxxxx)
    Affected if Any current version of Microsoft 365 Apps for enterprise or consumer Excel is listed as affected (all versions)
  4. Identify Office Online Server version (if applicable)
    On the server running Office Online Server, run 'Get-OfficeWebAppsServer' in PowerShell or check the installed version via Programs and Features. The version can also be checked in the registry at HKLM\Software\Microsoft\Office\WebServer\16.0\Version
    Affected if The installed version is lower than 16.0.10417.20083, indicating the server is vulnerable
  5. Confirm the vulnerability trigger condition
    This integer underflow vulnerability is triggered when Excel parses a specially crafted Excel file (.xlsx, .xls, .xlsm). The flaw occurs during file data processing, not through default Excel functionality alone
    Affected if Excel is installed and running any affected version as above; exploitation requires opening a maliciously crafted Excel file from an untrusted source

A system is affected if Microsoft Excel, Microsoft Office, or Microsoft Office Online Server is installed and the installed version falls within the affected ranges: Excel 2016+, Office 2019+, Office LTSC 2021+, Office LTSC 2024+, or Office Online Server versions below 16.0.10417.20083, with Microsoft 365 Apps at any current version being affected.

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.

From vendor data
Upgrade available Upgrade to 16.0.10417.20083 or later
Fixed in 16.0.10417.20083
Interim mitigation

Apply the appropriate Microsoft security updates for Microsoft Office Excel as soon as they become available. Until patches are available, exercise caution when opening Excel files from untrusted sources.

Fix this in 365 Apps Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
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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-2026-20957 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