CVE-2026-97776: Microsoft Excel Buffer Overflow Vulnerability

CVE-2026-97776 Overview

CVE-2026-97776 is a heap-based buffer overflow [CWE-122] in Microsoft Office Excel that enables remote code execution. An unauthorized attacker can execute arbitrary code on a target system when a user opens a maliciously crafted Excel file delivered over a network. The vulnerability requires user interaction but no authentication, making it suitable for phishing and watering-hole campaigns targeting enterprise users.

Microsoft published the advisory on October 13, 2026. The flaw affects supported versions of Microsoft Office Excel. Successful exploitation yields full confidentiality, integrity, and availability impact on the compromised host within the context of the current user.

Critical Impact
Remote attackers can achieve code execution with the privileges of the Excel user by delivering a crafted spreadsheet, enabling initial access, lateral movement, and payload staging.

Affected Products

Microsoft Office Excel (see the Microsoft Security Update Guide for the authoritative version list)
Microsoft 365 Apps for Enterprise distributions that bundle Excel
Office Long-Term Servicing Channel (LTSC) builds containing the vulnerable Excel component

Discovery Timeline

2026-10-13 - CVE CVE-2026-97776 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-97776

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in the Excel file-parsing code path. Excel allocates a heap buffer sized from attacker-controlled fields inside a crafted workbook. When the parser copies subsequent record data into that buffer, the length check does not match the actual payload size, allowing adjacent heap memory to be overwritten.

An attacker who overwrites heap metadata or function pointers adjacent to the buffer can redirect execution flow. In Excel, this typically leads to arbitrary code execution within the Office process, bypassing script-based macro warnings because the primitive sits in native parsing code rather than VBA. Exploitation runs under the current user's token, so standard-user compromise is straightforward and privilege escalation becomes a follow-on step.

Root Cause

The root cause is improper validation of a size or offset field used to allocate or index a heap buffer during Excel document parsing. The parser trusts attacker-supplied length metadata and writes outside the allocated region, which is the defining pattern of [CWE-122] heap-based buffer overflow.

Attack Vector

Delivery is network-based through standard document distribution channels, including email attachments, SharePoint links, OneDrive shares, and web downloads. The user must open the crafted .xls, .xlsx, or related Excel file for exploitation to occur. Protected View reduces the attack surface but is not a complete mitigation if the user clicks Enable Editing.

The vulnerability manifests inside the native Excel parser. Refer to the Microsoft Security Update Guide for technical details on affected file formats and build numbers. No public proof-of-concept exploit code was available at the time of publication.

Detection Methods for CVE-2026-97776

Indicators of Compromise

Excel (EXCEL.EXE) spawning child processes such as cmd.exe, powershell.exe, rundll32.exe, mshta.exe, or wscript.exe shortly after opening an attachment
Unexpected network connections initiated by EXCEL.EXE to external hosts following document open
Crash reports or Windows Error Reporting entries referencing EXCEL.EXE with heap corruption or access violation signatures
Excel writing executable content (.exe, .dll, .ps1, .hta) to user-writable directories such as %TEMP%, %APPDATA%, or %PUBLIC%

Detection Strategies

Hunt for Office process lineage anomalies where Excel is the parent of script interpreters or living-off-the-land binaries
Alert on inbound email attachments with Excel file extensions sourced from untrusted senders, especially those using legacy binary formats
Correlate Excel process creation events with outbound HTTP, HTTPS, or SMB connections to new or low-reputation destinations

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rules that block Office applications from creating child processes and executable content
Collect Sysmon Event ID 1 (process creation), Event ID 3 (network connection), and Event ID 11 (file create) with Office binaries in scope
Forward Office telemetry and endpoint events to a centralized SIEM or data lake for cross-host correlation and retrospective hunting

How to Mitigate CVE-2026-97776

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide across all endpoints running Excel
Prioritize patching for users who routinely receive external spreadsheets, including finance, procurement, and executive assistants
Verify Microsoft 365 Apps update channels are current and that managed devices have received the October 2026 Office servicing payload

Patch Information

Microsoft released fixes through the standard Office servicing channels on October 13, 2026. Consult the Microsoft Security Update Guide for the authoritative list of fixed builds for Microsoft 365 Apps, Office LTSC, and standalone Office installations. Confirm deployment by checking the Excel File > Account > About Excel build string against the vendor's fixed-version table.

Workarounds

Enforce Protected View and Office File Block policy to open files from the internet and Outlook attachments in a sandboxed, read-only state
Configure Group Policy to block legacy Excel binary formats (.xls, .xlb, .xla) from untrusted locations when business workflows allow
Enable the ASR rule Block all Office applications from creating child processes (GUID D4F940AB-401B-4EFC-AADC-AD5F3C50688A) in enforcement mode
Deliver targeted user awareness guidance to reject unexpected spreadsheets and report suspicious attachments to the security team

bash

# Enable the Office child-process ASR rule in enforcement mode via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Verify ASR rule state
Get-MpPreference | Select-Object -ExpandProperty AttackSurfaceReductionRules_Ids
Get-MpPreference | Select-Object -ExpandProperty AttackSurfaceReductionRules_Actions