CVE-2026-97762: Microsoft Excel RCE Vulnerability

CVE-2026-97762 Overview

CVE-2026-97762 is an out-of-bounds write vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute arbitrary code over a network. The flaw is classified under [CWE-787] (Out-of-bounds Write) and requires user interaction, typically achieved by convincing a target to open a crafted spreadsheet file. Successful exploitation grants the attacker code execution in the context of the current user, which can lead to full system compromise when the user holds administrative privileges.

Critical Impact

An attacker who successfully exploits CVE-2026-97762 can execute arbitrary code on the target system through a malicious Excel document, resulting in high impact to confidentiality, integrity, and availability.

Affected Products

• Microsoft Office Excel (see the Microsoft Security Advisory CVE-2026-97762 for the full list of affected builds)
• Microsoft 365 Apps for Enterprise installations that include Excel
• Standalone Microsoft Office installations containing the affected Excel component

Discovery Timeline

• 2026-10-13 - CVE CVE-2026-97762 published to NVD
• 2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-97762

Vulnerability Analysis

The vulnerability is an out-of-bounds write condition within Microsoft Office Excel's file parsing logic. When Excel processes a specially crafted spreadsheet, it writes data past the bounds of an allocated buffer. This memory corruption can be shaped by an attacker to overwrite adjacent structures, control program flow, and ultimately execute attacker-supplied code.

Because the attack vector is network-based with low complexity and requires no privileges, delivery methods such as email attachments, cloud document links, or compromised internal file shares are viable. The attack requires user interaction, meaning a victim must open or preview the malicious file. Exploitation yields code execution in the context of the invoking user.

Root Cause

The root cause is improper validation of input sizes or indices while parsing elements inside an Excel document. The parser calculates or trusts a length value that extends beyond the bounds of the destination buffer, producing a classic [CWE-787] out-of-bounds write. The precise structure affected is not disclosed in the public advisory; refer to the Microsoft Security Advisory CVE-2026-97762 for vendor-authoritative details.

Attack Vector

An attacker crafts a malicious .xlsx, .xls, or related Excel-supported file and delivers it to the victim through phishing, drive-by download, or a shared collaboration platform. When the user opens the file in a vulnerable Excel build, the parser triggers the out-of-bounds write and the attacker's shellcode runs with the user's privileges. No authentication to the target system is required. See the vendor advisory for authoritative technical details, as no verified public proof-of-concept is currently available.

Detection Methods for CVE-2026-97762

Indicators of Compromise

• Unexpected child processes spawned by EXCEL.EXE, particularly scripting hosts such as powershell.exe, cmd.exe, wscript.exe, or mshta.exe.
• Excel process crashes or Windows Error Reporting entries referencing memory access violations shortly after opening a document.
• Outbound network connections initiated by Excel to untrusted hosts or newly registered domains after a document is opened.
• Suspicious Excel documents arriving from external senders with unusual embedded objects, OLE streams, or oversized records.

Detection Strategies

• Monitor process-lineage telemetry for EXCEL.EXE parent-child relationships that deviate from normal user behavior.
• Hunt for Office applications writing executables, scripts, or DLLs into user-writable paths such as %TEMP%, %APPDATA%, or Downloads.
• Deploy YARA or structural file-analysis rules at the email gateway to flag Excel documents containing malformed records consistent with memory-corruption triggers.

Monitoring Recommendations

• Centralize Office telemetry, process creation events (Windows Event ID 4688 and Sysmon Event ID 1), and EDR behavioral alerts for correlation.
• Alert on Excel loading unusual modules or invoking Win32 APIs commonly associated with shellcode execution.
• Review mail-flow logs for inbound Excel attachments from first-time senders and quarantine for sandbox detonation.

How to Mitigate CVE-2026-97762

Immediate Actions Required

• Apply the Microsoft security update for CVE-2026-97762 across all endpoints running Microsoft Office or Microsoft 365 Apps, prioritizing internet-facing and high-privilege users.
• Block or quarantine inbound Excel attachments from untrusted senders at the email security gateway pending patch deployment.
• Communicate to users the risk of opening unsolicited spreadsheet files and enforce Protected View for documents originating from the internet.

Patch Information

Microsoft has published remediation guidance in the Microsoft Security Advisory CVE-2026-97762. Administrators should consult the advisory for the exact KB article numbers, build numbers, and update channels applicable to their Office deployment, then deploy through Microsoft Update, Windows Server Update Services (WSUS), Microsoft Intune, or Configuration Manager.

Workarounds

• Enforce Protected View and Office Application Guard policies so untrusted documents open in an isolated sandbox.
• Disable or restrict legacy file formats and ActiveX content through Group Policy where business processes allow.
• Use Attack Surface Reduction (ASR) rules to block Office applications from creating child processes and from spawning executable content.

# Example: enable the ASR rule that blocks Office from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled