Microsoft Office Word: Stack-based buffer overflow enabling remote code execution

Summary

Microsoft Office Word contains a stack-based buffer overflow (CWE-121) that allows an unauthorized attacker to execute code over the network. The attacker needs user interaction, so a victim must open a crafted document, typically received as an email attachment or from a file share. The CVSS 3.1 base score is 8.8 (High) with high impact on confidentiality, integrity and availability. NVD analysis was pending at publication, and no exploitation or public proof-of-concept was found in the sources reviewed.

Affected

• Microsoft 365 Apps for Enterprise: 16.0.1 before the fixed build listed at https://aka.ms/OfficeSecurityReleases
• Office 365 for Mac: 1.0.0 before 16.115.26110837
• Office LTSC 2021: 16.0.1 before 16.0.14334.21048
• Office LTSC 2024: 16.0.0 before 16.0.17932.21140 (secondary source, to be confirmed by Microsoft)
• Office LTSC for Mac 2021 and 2024: before 16.115.26110837 (not confirmed by Microsoft in the sources reviewed)

Fixed in

• Office LTSC 2021: 16.0.14334.21048
• Office 365 for Mac: 16.115.26110837
• Microsoft 365 Apps for Enterprise: Unknown. Check https://aka.ms/OfficeSecurityReleases
• Office LTSC 2024: 16.0.17932.21140 (secondary source, to be confirmed)
• Office LTSC for Mac 2021/2024: Unknown (to be confirmed by Microsoft)

Workaround

• Unknown. No vendor-documented workaround was found; the vendor patch is the remedy.
• Interim measures (not vendor-sourced): restrict Office child-process creation with Microsoft Defender Attack Surface Reduction rules, keep Protected View enabled, and filter Office attachments at the mail gateway.

Exploitation

• Exploited in the wild: no evidence found in the sources reviewed. Not verified against the CISA KEV catalog.
• Public proof-of-concept: none found.
• CVE.org record: exploit maturity Unproven (E:U), report confidence Confirmed (RC:C).
• CISA SSVC: Exploitation none, Automatable no, Technical Impact total.

Our recommendation

• Install the November 2026 Microsoft Office security updates on all affected Office installations, prioritising Microsoft 365 and email-exposed systems.
• Verify the fixed builds against the MSRC Security Update Guide, as some fixed versions are not yet confirmed by Microsoft.
• Apply the Defender ASR rule for blocking Office child-process creation and keep Protected View enabled.
• Filter or sandbox inbound Office documents at the mail gateway and monitor for unusual Word process behaviour.
• Recheck the NVD and CISA KEV status, as the record was published recently and may change.

Public exploits iAttack code is publicly available (PoC on GitHub, Metasploit or Exploit-DB).

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

• Microsoft Security Update Guide: CVE-2026-109505 msrc.microsoft.com
• CVE.org record CVE-2026-109505 www.cve.org
• ZDI November 2026 Security Update Review www.zerodayinitiative.com
• Strix CVE-2026-109505 www.strix.ai
• OffSeq Radar: November 2026 Patch Tuesday radar.offseq.com

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 10 Nov 2026, 23:48.