CVE-2026-108428: Microsoft 365 Apps Buffer Overflow Flaw

CVE-2026-108428 Overview

CVE-2026-108428 is a heap-based buffer overflow vulnerability in Microsoft Office Word. The flaw allows an unauthorized attacker to execute arbitrary code over a network when a victim opens a crafted document. The vulnerability is tracked under [CWE-122: Heap-based Buffer Overflow] and affects multiple Microsoft Office product lines, including Microsoft 365 Apps and Office LTSC editions.

Microsoft published the advisory and corresponding security update through the Microsoft Security Response Center. The issue carries a network attack vector but requires user interaction, typically through opening a weaponized Word document delivered by email or hosted on an attacker-controlled resource.

Critical Impact
Successful exploitation allows remote code execution in the context of the current user, enabling attackers to install programs, modify data, or create new accounts with the victim's privileges.

Affected Products

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

Discovery Timeline

2026-10-15 - CVE-2026-108428 published to NVD
2026-10-20 - Last updated in NVD database

Technical Details for CVE-2026-108428

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in Microsoft Office Word's document parsing logic. Attackers craft a malicious Word file that triggers an out-of-bounds write on the heap when the file is processed. The condition corrupts adjacent heap structures and provides an attacker-controlled primitive sufficient for arbitrary code execution.

Because Word runs with the privileges of the logged-in user, successful exploitation grants the attacker the same rights. On systems where users operate with administrative privileges, the impact extends to full system compromise. Confidentiality, integrity, and availability are all affected.

User interaction is required. The target must open the crafted document or, in some configurations, preview it through a vulnerable component. The Preview Pane in Outlook has historically served as an exploitation surface for similar Word parsing bugs.

Root Cause

The root cause is improper validation of size or boundary values during heap allocation and copy operations in Word's file format parser. When the parser processes malformed structures within a .doc or .docx file, it writes data beyond the bounds of a dynamically allocated buffer, overwriting adjacent heap metadata or object pointers.

Attack Vector

The attack vector is network-based. Typical delivery methods include phishing emails with attached Word documents, links to documents hosted on compromised or attacker-owned infrastructure, and documents embedded in collaboration platforms. Once the victim opens the document, exploitation proceeds without further interaction.

No public proof-of-concept exploit is available at the time of writing, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Microsoft Security Update for CVE-2026-108428 for authoritative technical details.

Detection Methods for CVE-2026-108428

Indicators of Compromise

Unexpected WINWORD.EXE child processes spawning command interpreters such as cmd.exe, powershell.exe, or wscript.exe
Word processes writing executable files, DLLs, or scripts to user-writable directories like %APPDATA% or %TEMP%
Outbound network connections initiated directly by WINWORD.EXE to untrusted domains or IP addresses
Crash events or faulting module entries referencing Office components in the Windows Application Event Log

Detection Strategies

Monitor process-creation telemetry for anomalous child processes of Office applications and correlate with document open events
Inspect email gateways for Word attachments with malformed OLE structures or oversized embedded objects
Apply YARA rules targeting known malformed field structures in Office Open XML and legacy binary document formats
Alert on Office applications loading unsigned or unexpected DLLs from user-writable paths

Monitoring Recommendations

Enable detailed audit logging for Office applications and forward logs to a centralized SIEM for correlation
Track file-write operations by Office processes to detect staging of secondary payloads
Baseline normal Office network behavior and alert on deviations such as connections to new external hosts
Review Windows Defender Exploit Guard and Attack Surface Reduction rule telemetry for blocked Office child-process activity

How to Mitigate CVE-2026-108428

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-108428 across all affected endpoints
Prioritize patching for users who regularly open externally sourced documents, including executives, finance, HR, and support staff
Validate that Microsoft 365 Apps update channels are current and that Office 2021 and 2024 LTSC installations have received the corresponding security update
Block or quarantine inbound Word attachments from untrusted senders at the email gateway pending remediation

Patch Information

Microsoft has released security updates addressing CVE-2026-108428. Administrators should consult the Microsoft Security Update Guide entry for CVE-2026-108428 for the specific build numbers applicable to each affected product. Microsoft 365 Apps installations receive updates through the configured update channel, while Office 2021 and 2024 LTSC require the appropriate MSI or Click-to-Run package.

Workarounds

Enable Protected View for documents originating from the internet and other untrusted locations
Enforce the Office Block macros from running in files from the Internet Group Policy setting
Deploy Attack Surface Reduction rules that block Office applications from creating child processes and from injecting code into other processes
Disable the Outlook Preview Pane for high-risk user groups until patches are fully deployed

bash

# Example: Enable ASR rule to block Office from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled