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PatchSiren cyber security CVE debrief

CVE-2026-40359 Microsoft CVE debrief

A use-after-free vulnerability in Microsoft Office Excel enables local code execution by an unauthorized attacker. The flaw (CWE-416) affects multiple Office deployment channels including Microsoft 365 Apps Enterprise, Office 2016, 2019, and Long Term Servicing Channel versions 2021 and 2024 across Windows (x64/x86) and macOS platforms, as well as Office Online Server prior to version 16.0.10417.20128. The CVSS 3.1 vector (AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) indicates local attack vector with low complexity, requiring no privileges but user interaction, yielding high impact on confidentiality, integrity, and availability. Published 2026-05-12 and last modified 2026-05-19. No KEV listing or known ransomware campaign use is documented.

Vendor
Microsoft
Product
Microsoft 365 Apps for Enterprise
CVSS
HIGH 7.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-12
Original CVE updated
2026-05-19
Advisory published
2026-05-12
Advisory updated
2026-05-19

Who should care

Organizations with Microsoft Office deployments, particularly those using Office Online Server or managing enterprise Office installations. Security teams responsible for endpoint protection and patch management. Users handling documents from external or untrusted sources.

Technical summary

The vulnerability stems from improper memory management in Excel where a freed object is subsequently referenced, allowing an attacker to corrupt memory and execute arbitrary code in the context of the current user. Attack requires local access and user interaction, typically through opening a maliciously crafted document. Affects broad Office install base including LTSC 2021/2024, Microsoft 365 Apps Enterprise, Office 2016/2019, and Office Online Server installations.

Defensive priority

HIGH

Recommended defensive actions

  • Apply security updates from Microsoft for affected Office versions per vendor advisory guidance
  • Prioritize patching for systems running Office Online Server and enterprise-managed Office deployments
  • Review and restrict macro execution policies and document handling from untrusted sources
  • Monitor for anomalous Excel process behavior indicating potential exploitation attempts
  • Validate endpoint detection coverage for memory corruption indicators in Office applications

Evidence notes

Vulnerability classification and affected product enumeration derived from NVD CPE criteria and Microsoft MSRC reference. CVSS scoring and CWE-416 classification sourced from official vulnerability database records.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-40359 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-40359

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

  • CVE-2026-40359 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-40359

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

Supplemental references

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.