CVE-2026-64903: Microsoft Office RCE Vulnerability

CVE-2026-64903 Overview

CVE-2026-64903 is an integer overflow vulnerability in Microsoft Office that allows an unauthorized attacker to execute code locally. The flaw is classified under [CWE-122] (Heap-based Buffer Overflow), where an integer wraparound leads to an undersized heap allocation followed by out-of-bounds writes. Exploitation requires user interaction, typically through opening a crafted Office document. Successful exploitation grants the attacker code execution in the context of the current user.

Critical Impact
Attackers can achieve arbitrary code execution on the local system with the privileges of the user who opens a malicious Office file, compromising confidentiality, integrity, and availability.

Affected Products

Microsoft Office (specific versions listed in the Microsoft Security Update CVE-2026-64903)
Refer to the Microsoft Security Response Center advisory for the authoritative product and build list
No CPE entries were published in the NVD record at the time of writing

Discovery Timeline

2026-08-11 - CVE-2026-64903 published to NVD
2026-08-12 - Last updated in NVD database

Technical Details for CVE-2026-64903

Vulnerability Analysis

The vulnerability originates from an integer overflow or wraparound during memory size calculations inside Microsoft Office parsing routines. When Office processes a crafted document, an arithmetic operation on attacker-controlled length or count fields wraps around the maximum integer boundary. The result is a heap allocation smaller than the data subsequently written into it.

Because the allocation is undersized, subsequent copy operations overflow the heap buffer. This corrupts adjacent heap metadata and object pointers. Attackers can shape the heap to place controlled data into predictable positions, then leverage the overflow to hijack execution flow.

The attack vector is local, meaning the crafted document must be delivered to and opened by a user. Common delivery methods include phishing email attachments, malicious links, and file shares. Once the document is opened, the vulnerable parser processes the malformed content without any additional privileges required from the attacker.

Root Cause

The root cause is missing or incorrect validation of arithmetic operations on size fields prior to heap allocation. When large or negative values combine, the computed size wraps, and the subsequent malloc-style call returns a buffer too small for the payload. This condition maps directly to [CWE-122] Heap-based Buffer Overflow, triggered by an upstream integer overflow.

Attack Vector

An attacker crafts a malicious Office document containing structures that trigger the integer overflow during parsing. The attacker delivers the file through email, web download, or removable media. When the victim opens the file in a vulnerable Office application, the parser miscalculates buffer size and writes controlled data past the allocation boundary, resulting in local code execution under the user's account.

No verified public exploit code is available. See the Microsoft Security Update CVE-2026-64903 advisory for vendor technical details.

Detection Methods for CVE-2026-64903

Indicators of Compromise

Unexpected child processes spawned by Office applications such as WINWORD.EXE, EXCEL.EXE, or POWERPNT.EXE (for example, cmd.exe, powershell.exe, rundll32.exe)
Office processes writing executable files or scripts to disk in user-writable directories such as %TEMP% or %APPDATA%
Outbound network connections initiated directly by Office processes to unfamiliar hosts
Crash events referencing Office modules with heap corruption signatures in Windows Error Reporting

Detection Strategies

Deploy behavioral endpoint identification rules that flag Office applications spawning script interpreters or shells
Hunt for anomalous DLL loads and injected threads within Office processes
Correlate document open events with subsequent process creation and network activity to expose exploitation chains
Inspect inbound email attachments for Office file formats containing malformed OLE or OOXML structures

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction rules that block Office child process creation
Forward Sysmon Event ID 1 (process creation) and Event ID 11 (file create) from Office processes to a central data lake for hunting
Alert on unsigned or newly observed binaries executed following an Office document open
Track Protected View bypass attempts and macro execution telemetry across the fleet

How to Mitigate CVE-2026-64903

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-64903 advisory to all affected Office installations
Prioritize patch deployment for endpoints belonging to users who regularly handle external documents
Enforce Protected View for files originating from the internet and email attachments
Restrict macro execution to signed macros from trusted publishers

Patch Information

Microsoft has issued a security update addressing CVE-2026-64903. Administrators should consult the Microsoft Security Update CVE-2026-64903 advisory for the complete list of affected builds and corresponding update packages. Deploy the update through Windows Update, Microsoft Update, WSUS, Intune, or Configuration Manager depending on your management tooling.

Workarounds

Block or quarantine legacy Office binary formats (.doc, .xls, .ppt) at the email gateway when business processes allow
Enable Attack Surface Reduction rule GUID D4F940AB-401B-4EFC-AADC-AD5F3C50688A to block Office applications from creating child processes
Open untrusted documents in Office for the web or a sandboxed environment until patching completes
Disable ActiveX controls and OLE object activation via Group Policy for high-risk user populations

bash

# Enable ASR rule to block Office child process creation via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled