CVE-2026-44818: Microsoft Excel RCE Vulnerability

CVE-2026-44818 Overview

CVE-2026-44818 is an integer underflow (wrap or wraparound) vulnerability in Microsoft Office Excel. The flaw allows an unauthorized attacker to execute code locally on a target system after a user opens a crafted Excel file. Microsoft published the advisory through the Microsoft Security Update Guide.

The vulnerability is classified under [CWE-362] and requires user interaction with low attack complexity offset by specific conditions. No public exploit or in-the-wild activity has been reported at the time of publication.

Critical Impact

Successful exploitation results in arbitrary code execution in the context of the user opening a malicious Excel document, leading to full confidentiality, integrity, and availability impact on the host.

Affected Products

• Microsoft Office Excel (refer to the Microsoft Security Update Guide for affected build numbers)

Discovery Timeline

• 2026-06-09 - CVE-2026-44818 published to the National Vulnerability Database
• 2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-44818

Vulnerability Analysis

The vulnerability stems from an integer underflow condition within Microsoft Office Excel's processing logic. When Excel parses a specifically crafted spreadsheet, an arithmetic operation produces a value that wraps below zero. The resulting unsigned value is interpreted as an unexpectedly large quantity, corrupting subsequent memory operations.

Exploitation requires a local attack vector and user interaction. A target must open a malicious workbook delivered through phishing, file shares, or removable media. The high attack complexity reflects conditions outside the attacker's control, such as specific Excel configurations or memory layouts required for reliable exploitation.

When triggered, the underflow corrupts memory in a way that allows attacker-controlled code to execute in the user's security context. Impact on confidentiality, integrity, and availability is high.

Root Cause

The root cause is an arithmetic boundary error in Excel's file parsing code. The application performs a subtraction or decrement operation on a value without verifying that the result remains within expected bounds. The wraparound produces a corrupted size or index value used later in memory allocation or buffer access routines.

Attack Vector

An attacker crafts an Excel document containing malformed structures designed to trigger the underflow. The attacker delivers the file to a victim through email, web download, or shared storage. Code execution occurs when the victim opens the document in a vulnerable version of Excel. The vulnerability does not require authentication on the target system but does require the victim to interact with the file.

No verified proof-of-concept code is publicly available. Refer to the Microsoft Security Update Guide for vendor-supplied technical details.

Detection Methods for CVE-2026-44818

Indicators of Compromise

• Unexpected child processes spawned by EXCEL.EXE, such as cmd.exe, powershell.exe, wscript.exe, or rundll32.exe
• Excel processes writing executable files or scripts to disk in user-writable locations such as %TEMP% or %APPDATA%
• Outbound network connections initiated directly from EXCEL.EXE to untrusted hosts shortly after a document is opened

Detection Strategies

• Hunt for Office application process trees that deviate from baseline behavior, focusing on script interpreters and LOLBins launched by Excel
• Inspect inbound email attachments and downloaded files for Excel workbooks containing malformed records or anomalous structures
• Correlate document open events with subsequent process creation, file write, and network activity within short time windows

Monitoring Recommendations

• Enable Microsoft Defender Attack Surface Reduction rules that block Office applications from creating child processes
• Forward Sysmon and Windows Security event logs to a centralized SIEM for cross-host correlation
• Track Office click-to-run version and patch level across the fleet to identify hosts still exposed to CVE-2026-44818

How to Mitigate CVE-2026-44818

Immediate Actions Required

• Apply the Microsoft security update referenced in the official advisory to all systems running Microsoft Office Excel
• Prioritize patching for users who routinely process Excel files from external sources
• Verify that Office click-to-run auto-update is enabled and functioning across managed endpoints

Patch Information

Microsoft has released a security update addressing CVE-2026-44818. Administrators should consult the Microsoft Security Update Guide for the exact KB articles and build numbers corresponding to their Office channel and version. Deploy the update through Microsoft Update, WSUS, Intune, or Configuration Manager as appropriate for the environment.

Workarounds

• Configure Excel to open documents from the internet in Protected View and block macros from internet-sourced files via Group Policy
• Restrict execution of child processes from Office applications using Attack Surface Reduction rules until patches are deployed
• Train users to avoid opening unsolicited Excel attachments and to validate the source of spreadsheets before opening

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