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PatchSiren cyber security CVE debrief

CVE-2026-55021 Microsoft CVE debrief

A spoofing vulnerability exists in Microsoft SharePoint Server, which could allow an authorized attacker to perform spoofing over a network. This issue is rated as HIGH with a CVSS score of 7.3. The vulnerability exists due to improper neutralization of input during web page generation. Affected versions include Microsoft SharePoint Enterprise Server 2016, Microsoft SharePoint Server 2019, and Microsoft SharePoint Server Subscription Edition. Defenders should verify and apply patches for affected versions to prevent potential spoofing attempts.

Vendor
Microsoft
Product
Microsoft SharePoint Enterprise Server 2016
CVSS
HIGH 7.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-14
Original CVE updated
2026-10-08
Advisory published
2026-07-14
Advisory updated
2026-10-08

Who should care

Defenders responsible for Microsoft SharePoint Server deployments should assess exposure and prioritize patching affected versions to prevent potential spoofing attempts.

Why it matters

A spoofing vulnerability in Microsoft SharePoint Server could allow an authorized attacker to perform spoofing over a network, emphasizing the need for defenders to verify and apply patches, monitor for spoofing attempts, and review server configurations.

  • Verify and apply patches for affected Microsoft SharePoint Server versions to prevent exploitation
  • Monitor for potential spoofing attempts to detect possible attacks
  • Review and update SharePoint Server configurations to ensure proper security settings

Technical summary

The vulnerability exists due to improper neutralization of input during web page generation, allowing an authorized attacker to perform spoofing over a network. Affected versions include Microsoft SharePoint Enterprise Server 2016, Microsoft SharePoint Server 2019, and Microsoft SharePoint Server Subscription Edition.

Defensive priority

Defenders should prioritize verifying and applying patches for affected Microsoft SharePoint Server versions, as well as monitoring for potential spoofing attempts.

Recommended defensive actions

  • Verify and apply patches for affected Microsoft SharePoint Server versions
  • Monitor for potential spoofing attempts
  • Review and update SharePoint Server configurations to ensure proper security settings

Evidence notes

The CVE Program record and NVD vulnerability detail page provide official information about the vulnerability. Microsoft has released a patch for this issue, which is available in the affected versions' updates.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-55021 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-55021

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-55021 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-55021

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

  • Microsoft SharePoint Server Spoofing Vulnerability

    Unverified legacy reference

    URL: https://raw.githubusercontent.com/CVEProject/cvelistV5/main/cves/2026/55xxx/CVE-2026-55021.json

    cve_program_cvelist_v5

  • Source reference

    Unverified legacy reference

    URL: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55021

    Supplemental source - vendor-advisory, patch

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.