CVE-2026-21511
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 · uneditedDeserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.
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 confidenceThis is an insecure deserialization vulnerability in Microsoft Office Outlook. An unauthenticated attacker can exploit this flaw over a network to perform spoofing attacks by sending specially crafted malicious data that gets deserialized by Outlook, allowing the attacker to impersonate legitimate entities or communications.
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= 2019= 2021= 2024< 16.0.19127.20518= 2016= 2019= 2016CVSS 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
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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 checksWork through these to decide whether this CVE applies to you.
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Check if Microsoft Outlook is installedOpen 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 applicationsAffected if Outlook is not present in the installed programs list - then the system is not affected by this Outlook vulnerability
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Determine the installed Outlook versionOpen Outlook, go to File > Office Account > About Outlook, or check the version in the Outlook executable properties at C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE (path may vary by version)Affected if The displayed version matches any of the affected versions: Microsoft 365 Apps (all versions), Office 2019, LTSC 2021, LTSC 2024, or the specific SharePoint/Word versions listed
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Check Outlook version via registryRun 'Get-ItemProperty HKLM:\Software\Microsoft\Office\ClickToRun\Configuration' or 'HKLM:\Software\Microsoft\Office\16.0\Common\InstallRoot' in PowerShell to find the Outlook version numberAffected if The VersionToReport or other version field falls within affected ranges (for Microsoft 365 all versions are affected; for Office 2019/2016/LTSC check against the specific versions listed)
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Verify if Outlook is configured for network connectivityCheck if Outlook profiles are configured to connect to Exchange, Microsoft 365, or other network servers. Open Outlook > File > Account Settings to see configured email accountsAffected if Outlook is configured with email accounts - the vulnerability is exploitable over the network when receiving specially crafted data
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Check SharePoint Server version if applicableCheck the SharePoint version via Central Administration > System Settings > Manage servers in this farm, or check the registry at HKLM\SOFTWARE\Microsoft\Shared Tools\ProductVersion\SharePoint for the version stringAffected if SharePoint version is less than 16.0.19127.20518, or equals 2016, or equals 2019 - these versions are listed as affected
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Check Microsoft Word version if applicableOpen Word > File > Account > About Word, or check the Word executable version at C:\Program Files\Microsoft Office\root\Office16\WINWORD.EXEAffected if Word version equals 2016 - this specific version is listed as affected, and the vulnerability is in the Outlook deserialization component that may be invoked by Word
A system is affected if it runs any version of Microsoft 365 Apps with Outlook, or specifically Outlook 2019, LTSC 2021, LTSC 2024, SharePoint Server 2016/2019 or before version 16.0.19127.20518, or Word 2016, and the Outlook component is installed and configured for email/network use.
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.
dbcve · scoped16.0.19127.20518
Apply the Microsoft security update for CVE-2026-21511 via Windows Update or enterprise patch management systems. Verify that all Outlook clients are patched to prevent deserialization attacks.
Microsoft 365 Apps (current channel or monthly enterprise channel); Office LTSC 2024 latest cumulative update; SharePoint Server builds >= 16.0.19127.20518
- Ensure Microsoft 365 Apps are updated through the cloud update channel or manually trigger updates via File > Account > Update Options > Update Now in any Office application
- For standalone Office installations (2019, 2021, 2024 LTSC), open any Office app, go to File > Account, and click Check for Updates to download the latest patches
- Verify the installed version by going to File > Account > About [App] and confirm the build number matches or exceeds the fixed release
- For SharePoint Server deployments, apply the cumulative update that includes version 16.0.19127.20518 or later from the Microsoft Update Catalog
- After applying updates, restart all Microsoft Office applications and verify Outlook launches without errors
- If automatic updates are disabled, enable them via Group Policy: Computer Configuration > Administrative Templates > Microsoft Office Updates > Enable Automatic Updates
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-21511 — 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-2026-21511 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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- 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