CVE-2026-97756: Microsoft Excel Buffer Overflow Vulnerability

CVE-2026-97756 Overview

CVE-2026-97756 is a heap-based buffer overflow vulnerability in Microsoft Office Excel. An unauthorized attacker can execute arbitrary code over a network by crafting a malicious Excel file and convincing a user to open it. The flaw is classified under [CWE-122: Heap-based Buffer Overflow] and affects the integrity, confidentiality, and availability of the target system.

Microsoft published the advisory on October 13, 2026. Successful exploitation grants code execution in the context of the user running Excel, which commonly serves as an initial access vector in phishing and targeted intrusion campaigns.

Critical Impact
An attacker who successfully exploits this vulnerability can execute arbitrary code in the context of the current user via a crafted Excel document delivered through phishing or web distribution.

Affected Products

Microsoft Office Excel (version details available in the Microsoft Security Response Center advisory)
Office installations receiving updates through the October 2026 security release
Any Office configuration that opens untrusted spreadsheets outside Protected View

Discovery Timeline

2026-10-13 - CVE CVE-2026-97756 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-97756

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in Microsoft Office Excel's file parsing logic. When Excel processes a malformed spreadsheet, it writes attacker-controlled data past the bounds of a heap-allocated buffer. This corruption can overwrite adjacent memory structures, including object pointers and function tables used by the application.

Exploitation leads to remote code execution in the security context of the user who opens the document. Because Excel is deployed broadly across enterprises, the vulnerability is well-suited to initial access operations and lateral delivery through compromised mailboxes.

The attack requires user interaction, meaning the victim must open the malicious workbook. The EPSS probability stands at approximately 0.80%, reflecting a measurable likelihood of exploitation attempts following patch release.

Root Cause

The root cause is improper validation of input length or structure during Excel's parsing of spreadsheet records. The parser allocates a heap buffer sized for expected content but fails to enforce bounds when copying attacker-controlled data from the file. This mismatch allows heap corruption that an attacker can shape into controlled code execution.

Attack Vector

The attack vector is network-based, with delivery typically occurring through email attachments, cloud file shares, or web downloads. The attacker crafts a malicious .xlsx, .xls, or related Excel file and delivers it to the target. When the user opens the file in a vulnerable Excel version, the parsing routine triggers the heap overflow. No authentication is required on the target system. Refer to the Microsoft Security Update for CVE-2026-97756 for file format specifics.

No verified public proof-of-concept code is currently available. The vulnerability mechanism involves out-of-bounds heap writes during document parsing, consistent with other historical Office file format vulnerabilities.

Detection Methods for CVE-2026-97756

Indicators of Compromise

Unexpected child processes spawned by excel.exe, such as cmd.exe, powershell.exe, rundll32.exe, or mshta.exe
Excel process crashes with heap corruption exceptions (0xC0000374, 0xC0000005) when opening untrusted files
Outbound network connections initiated by excel.exe to unfamiliar domains or IP addresses shortly after a document is opened
Creation of persistence artifacts (scheduled tasks, Run keys, startup folder entries) following Excel document access

Detection Strategies

Monitor process lineage for excel.exe spawning scripting engines or living-off-the-land binaries
Alert on Excel writing executable files (.exe, .dll, .scr) to disk, especially in user-writable paths
Inspect inbound email attachments and web downloads for Excel files sourced from external senders
Correlate Excel crash telemetry with file-open events to surface exploitation attempts

Monitoring Recommendations

Enable Microsoft Defender attack surface reduction (ASR) rules that block Office child process creation
Forward Office telemetry and Windows Event Logs to a centralized SIEM for retention and correlation
Track Office version inventory across endpoints to confirm patch deployment coverage
Monitor for anomalous DLL loads into excel.exe that originate outside the Office install directory

How to Mitigate CVE-2026-97756

Immediate Actions Required

Apply the October 2026 Microsoft security update for Office Excel as referenced in the Microsoft Security Response Center advisory
Confirm Protected View is enforced for files originating from the internet and email attachments
Review mail gateway rules to quarantine Excel files from untrusted external senders
Validate ASR rules blocking Office applications from launching child processes are enabled in Block mode

Patch Information

Microsoft released a security update addressing CVE-2026-97756 on October 13, 2026. Administrators should deploy the update through Microsoft Update, WSUS, Intune, or Microsoft 365 Apps update channels. Full version mapping and KB article identifiers are published in the Microsoft Security Update guide for CVE-2026-97756.

Workarounds

Enforce Protected View and Office File Block policies to prevent Excel from parsing legacy or untrusted file formats
Disable macro execution for files from the internet using Group Policy or Microsoft 365 administrative controls
Route Excel attachments through a sandbox detonation service before delivery to end users
Restrict end-user permissions so that code execution within Excel cannot escalate to administrative actions

bash

# Example: Enforce Protected View via Group Policy registry keys
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 DisableInternetFilesInPV /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