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PatchSiren cyber security CVE debrief

CVE-2026-40421 Microsoft CVE debrief

A medium-severity information disclosure vulnerability in Microsoft Office Word, published 2026-05-12 and last modified 2026-05-19. The flaw stems from external control of file name or path (CWE-73), allowing an unauthenticated remote attacker to disclose information over a network when a user interacts with a malicious document. CVSS 3.1 score of 4.3 reflects network attack vector, low attack complexity, required user interaction, and low confidentiality impact with no integrity or availability impact. Affected products include Microsoft 365 Apps for Enterprise (x64/x86), Office 2019/2024/LTSC 2021 (x64/x86), and Word 2016 (x64/x86). No known exploitation in the wild or ransomware campaign use has been reported; the vulnerability is not listed in CISA KEV.

Vendor
Microsoft
Product
Microsoft 365 Apps for Enterprise
CVSS
MEDIUM 4.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-12
Original CVE updated
2026-06-01
Advisory published
2026-05-12
Advisory updated
2026-06-01

Who should care

Organizations running Microsoft Office 2016, 2019, 2021 LTSC, 2024 LTSC, or Microsoft 365 Apps for Enterprise; security teams responsible for endpoint protection and patch management; users handling documents from external or untrusted sources

Technical summary

The vulnerability exists in Microsoft Office Word's handling of file names or paths, where insufficient validation allows an attacker to influence file operations. When a user opens a crafted document, the application may access attacker-controlled paths, leading to information disclosure over the network. The attack requires user interaction (opening a malicious file) but does not require privileges. The confidentiality impact is rated low per CVSS, with no integrity or availability impact. Multiple Office and Word versions across x86 and x64 architectures are affected.

Defensive priority

medium

Recommended defensive actions

  • Apply security updates from Microsoft as referenced in the vendor advisory for affected Office and Word versions
  • Restrict macro execution and external content in Word via Group Policy or application settings
  • Educate users on phishing risks and safe handling of documents from untrusted sources
  • Monitor for suspicious network activity from Office applications indicating potential information exfiltration

Evidence notes

Vulnerability description and CVSS vector sourced from NVD record with vendor advisory from Microsoft Security Response Center. CWE-73 classification confirmed via NVD weaknesses field. Affected product list derived from CPE criteria in NVD source data. No KEV entry present per source enrichment data.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-40421 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-40421

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

  • CVE-2026-40421 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-40421

    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.