CVE-2026-100210: Microsoft 365 Apps Buffer Overflow

CVE-2026-100210 Overview

CVE-2026-100210 is a heap-based buffer overflow [CWE-122] in Microsoft Office Excel that enables remote code execution. An unauthorized attacker can execute arbitrary code when a user opens a crafted Excel file delivered over a network. The flaw affects multiple Microsoft Office products including Microsoft 365 Apps, Office 2021, Office 2024, and Office Online Server across Windows and macOS platforms.

Successful exploitation grants the attacker code execution in the context of the current user. On systems where Excel runs with elevated privileges, this can lead to full system compromise and lateral movement across enterprise environments.

Critical Impact
Attackers can achieve arbitrary code execution with user-level privileges by convincing a target to open a malicious Excel document, enabling malware deployment, credential theft, and persistent access.

Affected Products

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

Discovery Timeline

2026-10-13 - CVE-2026-100210 published to the National Vulnerability Database (NVD)
2026-10-15 - Last updated in NVD database

Technical Details for CVE-2026-100210

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in the Excel parsing component. Excel allocates a heap buffer to hold structured spreadsheet data, but fails to properly validate the size of attacker-controlled input before copying it into that buffer. The resulting out-of-bounds write corrupts adjacent heap memory.

Attackers craft malicious Excel files containing malformed records that trigger the overflow during parsing. Because the corruption occurs on the heap, adversaries can shape memory layout to overwrite function pointers, virtual tables, or object metadata. This enables hijacking of execution flow and arbitrary code execution within the Excel process.

Exploitation requires user interaction. The target must open the malicious document, typically delivered through phishing, a watering hole, or file-sharing services. The network attack vector reflects remote delivery of the payload rather than remote triggering without user action.

Root Cause

The root cause is insufficient bounds checking during heap allocation and buffer population when Excel processes crafted file structures. The parser trusts length or offset fields from untrusted input, leading to a classic [CWE-122] heap-based buffer overflow.

Attack Vector

The attacker delivers a weaponized .xlsx, .xls, or related Excel file to the victim through email, web download, or shared network location. When the file is opened in a vulnerable version of Excel, the parsing routine processes the malformed structure and triggers the overflow. Preview Pane rendering may also expose the vulnerability depending on configuration.

The vulnerability manifests during document parsing. No verified public proof-of-concept is available. Refer to the Microsoft Security Update CVE-2026-100210 advisory for additional technical details.

Detection Methods for CVE-2026-100210

Indicators of Compromise

Unexpected child processes spawned by EXCEL.EXE, especially cmd.exe, powershell.exe, wscript.exe, or rundll32.exe
Excel process crashes with heap corruption exceptions in Windows Event Logs
Outbound network connections from EXCEL.EXE to untrusted or newly registered domains
Creation of executable files or scripts in user-writable directories by the Excel process

Detection Strategies

Monitor for anomalous process trees where Office applications launch scripting engines or living-off-the-land binaries (LOLBins)
Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office child process creation and executable content
Hunt for Excel files sourced from external email with embedded objects, macros, or malformed structures using sandbox analysis

Monitoring Recommendations

Collect and centralize Office telemetry including process creation, image loads, and file writes for behavioral analysis
Flag Excel instances that load unusual DLLs or allocate large contiguous heap regions prior to crashing
Correlate email gateway attachment metadata with endpoint document-open events to trace delivery paths

How to Mitigate CVE-2026-100210

Immediate Actions Required

Apply the Microsoft security update referenced in the MSRC advisory for all affected Office versions
Prioritize patching for users who routinely handle externally sourced spreadsheets, including finance, procurement, and executive staff
Enable Protected View and Office Application Guard to isolate documents from untrusted sources
Deploy ASR rules to block Office applications from spawning child processes and creating executable content

Patch Information

Microsoft has released security updates addressing CVE-2026-100210 across Microsoft 365 Apps, Office 2021 LTSC, Office 2024 LTSC, and Office Online Server. Administrators should consult the Microsoft Security Update Guide for the exact build numbers and deployment packages for each product channel and platform.

Workarounds

Block inbound Excel attachments at the email gateway or route them through a detonation sandbox before delivery
Configure Office File Block policy to prevent opening legacy Excel binary formats from untrusted locations
Restrict Excel from opening files originating from the Internet Zone using Group Policy and Mark-of-the-Web enforcement
Educate users to avoid opening unsolicited spreadsheets and to report suspicious attachments

bash

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

# Example: Enforce Protected View via Group Policy registry setting
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" `
    /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f