CVE-2026-78507: Microsoft 365 Apps Use After Free Flaw

CVE-2026-78507 Overview

CVE-2026-78507 is a use-after-free vulnerability [CWE-416] in Microsoft Office Word that enables remote code execution. An attacker can craft a malicious document that, when opened by a user, triggers memory corruption and executes arbitrary code in the context of the current user. The flaw affects multiple supported Office releases across Windows and macOS platforms.

Microsoft published the advisory through the Microsoft Security Response Center. Successful exploitation requires user interaction, typically opening or previewing a weaponized .docx or .rtf file delivered via email, web download, or file share.

Critical Impact
Remote code execution in the user context, enabling malware deployment, credential theft, and lateral movement from a single opened document.

Affected Products

Microsoft 365 Apps (x64 and x86 Enterprise editions)
Microsoft Office 2019, Office LTSC 2021, Office LTSC 2024 (Windows and macOS)
Microsoft 365 for macOS

Discovery Timeline

2026-09-08 - CVE-2026-78507 published to the National Vulnerability Database
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-78507

Vulnerability Analysis

The vulnerability is a use-after-free condition in Microsoft Office Word's document parsing logic. Word references a memory object after it has already been freed, allowing an attacker to reclaim that memory region with controlled data. When the dangling pointer is dereferenced, the attacker-controlled data influences program flow, leading to arbitrary code execution.

Exploitation requires the target to open a specially crafted document. Because Word rendering occurs in the user's session, executed code inherits the user's privileges. On endpoints where users hold local administrator rights, an attacker gains full control of the host.

The attack vector is network-based, meaning malicious files can be delivered through phishing emails, malicious websites, SharePoint links, or shared drives. No authentication is required on the target system beyond a user opening the file.

Root Cause

The root cause is improper object lifetime management within Word's document handling code. Specific object references persist after the underlying memory allocation has been released, violating the memory safety contract described by [CWE-416: Use After Free].

Attack Vector

An attacker crafts a Word document that manipulates internal object allocation and deallocation sequences during parsing. When the document is opened, Word frees an object while retaining a reference to it. The attacker then triggers a heap spray or controlled allocation to place attacker data at the freed address. Subsequent use of the stale pointer transfers execution to attacker-controlled memory.

The vulnerability mechanism follows a standard use-after-free exploitation pattern within Office document parsing. Refer to the Microsoft Security Update Guide for authoritative technical details.

Detection Methods for CVE-2026-78507

Indicators of Compromise

Unexpected child processes spawned by WINWORD.EXE, such as cmd.exe, powershell.exe, wscript.exe, mshta.exe, or rundll32.exe.
Word crash events in the Windows Application event log referencing faulting modules within wwlib.dll or related Office components.
Outbound network connections from WINWORD.EXE to unrecognized domains or IP addresses immediately after a document is opened.
Creation of executable files or scheduled tasks by Office processes within user profile directories.

Detection Strategies

Deploy behavioral detection rules that flag Office applications spawning script interpreters or LOLBins as a high-fidelity signal of document-based exploitation.
Hunt for anomalous memory allocation patterns and process injection originating from WINWORD.EXE using EDR telemetry.
Correlate email gateway attachment metadata with endpoint document-open events to identify weaponized document delivery.

Monitoring Recommendations

Enable Microsoft Office cloud protection and attack surface reduction (ASR) rules, particularly the rule blocking Office child process creation.
Forward Sysmon process creation, image load, and network connection events into a SIEM for correlation and retrospective hunting.
Monitor for Word application crashes across the fleet, as clusters of crashes may indicate exploitation attempts or unreliable exploit chains.

How to Mitigate CVE-2026-78507

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-78507 to all affected Office and Microsoft 365 Apps installations.
Confirm that Microsoft 365 Apps clients are on a supported update channel and receiving current builds via Click-to-Run.
Prioritize patching endpoints belonging to high-value users such as executives, finance, HR, and IT administrators, who are frequent phishing targets.

Patch Information

Microsoft has released fixes through its standard security update channels. Administrators should consult the Microsoft Security Update Guide for specific build numbers per product edition and deploy updates through Windows Update, Microsoft Update, WSUS, Intune, or Configuration Manager.

Workarounds

Enable Protected View for documents originating from the internet, email attachments, and unsafe locations to open untrusted files in a sandboxed process.
Configure Office File Block policies to prevent legacy binary formats such as .doc from opening or to force them through Protected View.
Enable ASR rules including "Block all Office applications from creating child processes" and "Block Win32 API calls from Office macros" via Microsoft Defender or Intune.
Restrict local administrator rights on end-user workstations to limit the blast radius of successful exploitation.

bash

# Example: Enable the ASR rule that blocks Office child process creation
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled