CVE-2026-108431: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-108431 Overview

CVE-2026-108431 is a stack-based buffer overflow [CWE-121] in Microsoft Office Word that allows an unauthenticated attacker to execute arbitrary code over a network. Exploitation requires a user to open or preview a crafted document. Successful attacks grant the adversary code execution in the context of the current user, enabling installation of malware, credential theft, and lateral movement. The vulnerability affects Microsoft 365 Apps, Microsoft 365 on macOS, Office LTSC 2021, and Office LTSC 2024 across x86, x64, and macOS editions.

Critical Impact
A single crafted Word document can trigger memory corruption on the stack and run attacker-controlled code with the privileges of the user opening the file.

Affected Products

Microsoft 365 Apps (Enterprise, x86 and x64)
Microsoft 365 for macOS
Microsoft Office LTSC 2021 and Office LTSC 2024 (Windows and macOS)

Discovery Timeline

2026-10-15 - CVE-2026-108431 published to the National Vulnerability Database
2026-10-20 - Last updated in NVD database
2026-10-22 - EPSS score recorded at 0.441%

Technical Details for CVE-2026-108431

Vulnerability Analysis

The vulnerability is a stack-based buffer overflow in the Word document parsing path. Word writes attacker-controlled data from a crafted file into a fixed-size stack buffer without validating length. The write overruns adjacent stack memory, including saved return addresses and structured exception handler records. An attacker who controls the overflow contents can hijack the control flow and execute arbitrary code in the user's security context.

Exploitation requires user interaction. A victim must open the crafted document or render it through a vulnerable preview path. Delivery vectors include email attachments, instant messaging payloads, and documents hosted on attacker-controlled web servers or SMB shares. No authentication to the target is required.

Root Cause

The defect falls under [CWE-121: Stack-based Buffer Overflow]. Office code accepts a length value or variable-length field from the document structure and copies the data into a stack buffer sized for an expected maximum. When the document specifies a larger value, the copy overflows the buffer and corrupts stack metadata. Mitigations such as Stack Canaries, Data Execution Prevention (DEP), and Address Space Layout Randomization (ASLR) raise the exploitation bar but do not eliminate risk, particularly when combined with information leaks or Return-Oriented Programming (ROP).

Attack Vector

The attack vector is network-based with user interaction. Common delivery paths include phishing email with a malicious .doc or .docx attachment, Office documents hosted on compromised SharePoint or WebDAV servers, and macro-free documents designed to trigger parsing before user approval. On macOS editions, exploitation targets the native Word for Mac binary. Code executes under the current user, so attackers often chain this flaw with local privilege escalation primitives to reach SYSTEM or root.

No public proof-of-concept exploit is listed in the vendor advisory, and the CVE is not present on the CISA Known Exploited Vulnerabilities catalog. Technical details and reproduction steps are available only through the Microsoft Security Update Guide.

Detection Methods for CVE-2026-108431

Indicators of Compromise

winword.exe or Microsoft Word (macOS) spawning command interpreters such as cmd.exe, powershell.exe, wscript.exe, bash, or osascript shortly after document open.
Unexpected child processes making outbound network connections to uncategorized or newly registered domains.
Office writing executable content (.exe, .dll, .scr, .hta) to %TEMP%, %APPDATA%, or ~/Library/Caches/.
Crash events for winword.exe with exception codes indicating stack corruption (0xC0000409, 0xC0000005) immediately after opening an email attachment.

Detection Strategies

Hunt for Word process ancestry violations where winword.exe is the parent of scripting engines, LOLBins (mshta.exe, regsvr32.exe, rundll32.exe), or curl/certutil invocations.
Alert on documents originating from Mark-of-the-Web (MOTW) sources that trigger process creation events.
Correlate email gateway telemetry on inbound .doc, .docx, .rtf, and .docm attachments with endpoint document-open events on the same host.

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and from writing executable content.
Forward Sysmon Event IDs 1 (process create), 3 (network connect), and 11 (file create) from Office hosts to a central analytics platform.
Monitor Protected View and Mark-of-the-Web bypass attempts in Office telemetry logs.

How to Mitigate CVE-2026-108431

Immediate Actions Required

Apply the October 2026 Microsoft security updates for all affected Office SKUs referenced in the Microsoft Security Update Guide for CVE-2026-108431.
Confirm Microsoft 365 Apps clients on Current Channel and Monthly Enterprise Channel have received the fixed build; audit Office LTSC 2021 and 2024 installations separately.
Block inbound Office documents from external senders at the mail gateway where business processes allow, and quarantine attachments carrying macros or legacy binary formats.
Reinforce user awareness that Protected View must not be disabled for documents received from the internet.

Patch Information

Microsoft has released security updates addressing CVE-2026-108431 across Microsoft 365 Apps (x86 and x64 Enterprise), Microsoft 365 for macOS, Office LTSC 2021, and Office LTSC 2024. Patch metadata, affected build numbers, and deployment guidance are published in the Microsoft Security Response Center advisory. Administrators managing LTSC installations should apply the appropriate MSI update packages, while Click-to-Run deployments update automatically once the channel is refreshed.

Workarounds

Enforce Protected View and Office Protected Mode via Group Policy for files originating from the internet, email attachments, and unsafe locations.
Configure the File Block policy to prevent opening legacy Word binary formats (.doc, .dot, .wbk) from untrusted sources.
Enable ASR rule Block all Office applications from creating child processes (GUID D4F940AB-401B-4EFC-AADC-AD5F3C50688A) in Block mode.

bash

# Enable ASR rule to block Office applications from creating child processes (PowerShell)
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Verify the rule is active
Get-MpPreference | Select-Object -ExpandProperty AttackSurfaceReductionRules_Ids