CVE-2026-97769: Microsoft Excel Buffer Overflow Vulnerability

CVE-2026-97769 Overview

CVE-2026-97769 is a heap-based buffer overflow vulnerability in Microsoft Office Excel. The flaw allows an unauthorized attacker to execute arbitrary code over a network when a user opens a crafted Excel document. Microsoft classifies the issue under [CWE-20] (Improper Input Validation), indicating that Excel fails to properly validate structured data within spreadsheet files before processing it in heap memory.

The vulnerability requires user interaction, typically through opening a malicious .xlsx, .xls, or related Office document delivered via phishing, shared drives, or web download. Successful exploitation yields code execution in the context of the user running Excel.

Critical Impact
Attackers who successfully exploit CVE-2026-97769 can execute code with the privileges of the current user, enabling payload delivery, credential theft, and lateral movement.

Affected Products

Microsoft Office Excel (specific builds listed in the Microsoft Security Response Center advisory)
Microsoft 365 Apps deployments that include Excel
Standalone Office installations where Excel processes untrusted spreadsheet files

Discovery Timeline

2026-10-13 - CVE-2026-97769 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-97769

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow triggered when Microsoft Excel parses specially crafted file content. Excel allocates a heap buffer sized according to attacker-controllable fields in the document. When the parser copies file data into that buffer, it writes past the allocated bounds and corrupts adjacent heap memory.

The attacker gains influence over heap metadata, function pointers, or object vtables stored near the overflow target. Chained with modern exploitation primitives such as heap grooming and information leaks, this corruption leads to arbitrary code execution in the Excel process.

The flaw is reachable over the network because malicious documents can be delivered via email attachments, SharePoint, OneDrive links, or any file transfer mechanism. Exploitation requires the user to open the document, which satisfies the user interaction requirement in the CVSS vector.

Root Cause

The root cause is improper input validation ([CWE-20]) during spreadsheet parsing. Excel does not sufficiently validate size or structural fields in the file format before using them to size or index a heap buffer. This creates the overflow condition when crafted values exceed what the allocation can hold.

Attack Vector

The attack vector is network-based with low complexity but requires user interaction. A typical exploitation chain begins with an attacker sending a weaponized Excel file to a target. When the user opens the file and Excel parses the malicious record, the heap overflow executes attacker-controlled code. See the Microsoft CVE-2026-97769 Advisory for technical details.

// No verified public proof-of-concept code is available for CVE-2026-97769.
// Refer to the Microsoft Security Response Center advisory for technical details.

Detection Methods for CVE-2026-97769

Indicators of Compromise

Excel (EXCEL.EXE) spawning child processes such as cmd.exe, powershell.exe, wscript.exe, or rundll32.exe
Unexpected network connections initiated by EXCEL.EXE to untrusted external hosts shortly after a document is opened
Crash events or Windows Error Reporting entries referencing heap corruption in Excel modules
Office documents delivered via email with macros disabled but containing anomalous embedded objects or external data references

Detection Strategies

Monitor process-creation telemetry for Excel spawning scripting or living-off-the-land binaries, which is a common post-exploitation pattern
Inspect email gateways and web proxies for Office file attachments with abnormal structure, oversized records, or malformed OLE streams
Correlate Excel crashes with subsequent process or network anomalies on the same host to surface failed or successful exploitation attempts

Monitoring Recommendations

Collect endpoint telemetry including process trees, module loads, and command-line arguments for all Office applications
Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and log violations centrally
Forward Windows Error Reporting and application crash logs to a centralized SIEM for correlation with user activity and file-open events

How to Mitigate CVE-2026-97769

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft CVE-2026-97769 Advisory across all systems running Microsoft Office Excel
Prioritize patching on endpoints used by high-risk roles such as finance, executive, and administrative users who routinely open external spreadsheets
Block inbound Excel attachments from untrusted senders at the email gateway until patch deployment is verified

Patch Information

Microsoft published patch guidance in the Microsoft CVE-2026-97769 Advisory. Administrators should deploy the update through Microsoft Update, Windows Server Update Services (WSUS), Microsoft Intune, or Microsoft 365 Apps update channels and confirm installation using update history and build version checks.

Workarounds

Enforce Protected View for files originating from the internet and other untrusted locations to prevent active parsing of malicious content
Enable Office File Block policies to prevent opening of legacy Excel binary formats where feasible
Deploy ASR rules that block Office child-process creation and Win32 API calls from Office macros to limit exploitation impact
Restrict Mark-of-the-Web stripping on inbound files so untrusted documents retain security zone metadata

bash

# Example: enforce Protected View via Group Policy registry keys on Windows endpoints
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f