CVE-2026-55021
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 · uneditedImproper neutralization of input during web page generation ('cross-site scripting') in Microsoft Office SharePoint allows an authorized attacker to perform spoofing over a network.
In the news
Third-party coverage- Microsoft July 2026 Patch Tuesday fixes massive 570 flaws, 3 zero-days
- Microsoft CVE Summary - Bleeping Computer
Surfaced from public web coverage — external links open in a new tab.
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 · high confidenceThis is a stored Cross-Site Scripting (XSS) vulnerability in Microsoft Office SharePoint where improper input validation allows an authenticated attacker to inject malicious scripts into SharePoint pages. The vulnerability enables spoofing attacks, meaning attackers can impersonate legitimate users or bypass security controls by executing arbitrary JavaScript in the context of other users' sessions.
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< 16.0.19725.20434= 2016= 2019CVSS 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
- Low
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- High
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/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 SharePoint Server build numberOpen SharePoint Management Shell and run: (Get-SPFarm).BuildVersion or access Central Administration > System Settings > Manage farm configuration to view the build versionAffected if The build number is less than 16.0.19725.20434, or the installed version is SharePoint Server 2016 or SharePoint Server 2019 (any build)
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Confirm SharePoint edition and versionRun: Get-SPServer | Select-Object * or check Central Administration > About SharePoint to identify if the installation is Server 2016 or Server 2019Affected if The edition is SharePoint Server 2016 or SharePoint Server 2019 regardless of build number
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Verify if web-facing input forms existReview published SharePoint sites for user input mechanisms such as list item forms, custom web parts, or third-party extensions that accept user-supplied contentAffected if Authenticated users can submit content through any SharePoint input field that lacks proper output encoding
The environment is affected if the SharePoint Server build is below 16.0.19725.20434, or if running SharePoint Server 2016 or 2019, and the server accepts authenticated user input through web forms or custom content submission points.
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 · scoped16.0.19725.20434
Apply the Microsoft security updates for SharePoint Server to properly neutralize user-supplied input. Verify that all SharePoint instances in the environment are patched and validate that input sanitization is functioning correctly.
Microsoft SharePoint Server Cumulative Update containing build 16.0.19725.20434 or later (e.g., July 2025 CU or subsequent)
- Check current SharePoint Server build version via SharePoint Administration Central or PowerShell (Get-SPFarm).
- Identify the required update: Microsoft SharePoint Server Cumulative Update (CU) containing version 16.0.19725.20434 or higher.
- Obtain the update from Microsoft Update Catalog or through your normal WSUS/SCCM update channel.
- Test the update in a non-production SharePoint farm environment before deploying to production.
- Schedule a maintenance window and apply the update following Microsoft documentation for your SharePoint version (2016, 2019).
- After installation, verify the build version has updated to 16.0.19725.20434 or later.
- Validate that SharePoint services and web applications are functioning correctly post-update.
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation6.0 h
- Implementation8.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $7,296.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-55021 — 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-2026-55021 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