CVE-2026-97763: Microsoft Excel Buffer Overflow Vulnerability

CVE-2026-97763 Overview

CVE-2026-97763 is a heap-based buffer overflow [CWE-122] in Microsoft Office Excel that allows an unauthorized attacker to execute code over a network. The flaw affects Excel's handling of specially crafted spreadsheet files. Successful exploitation requires user interaction, typically opening a malicious document delivered via email, file share, or web download. An attacker who successfully exploits this vulnerability can run arbitrary code in the context of the current user.

Critical Impact
Remote code execution in the context of the Excel user, enabling installation of malware, credential theft, and lateral movement across the enterprise.

Affected Products

Microsoft Office Excel (see Microsoft Security Update Guide for the authoritative product and build list)

Discovery Timeline

2026-10-13 - CVE CVE-2026-97763 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-97763

Vulnerability Analysis

The vulnerability is a heap-based buffer overflow in Microsoft Office Excel. Excel allocates a buffer on the heap when parsing structured data inside a spreadsheet, then writes attacker-controlled content past the allocated boundary. The out-of-bounds write corrupts adjacent heap metadata and object pointers.

An attacker who shapes the heap layout before the overflow can gain control of the instruction pointer. Code then executes with the privileges of the user who opened the file. Because Excel runs within the user session, the attacker inherits any permissions assigned to that account, including access to local files, mapped drives, and cached credentials.

The EPSS model estimates a 0.738% probability of exploitation within 30 days. Office document parsers remain a recurring target for commodity malware and targeted intrusion sets.

Root Cause

The root cause is improper validation of a length or size field during parsing of Excel file content. The parser trusts an attacker-supplied value and copies data into a heap buffer that is smaller than the input, violating memory safety guarantees described in [CWE-122].

Attack Vector

The attack vector is network delivery of a malicious Excel file. User interaction is required: the victim must open the crafted document or trigger a preview that invokes the vulnerable parser. Common delivery channels include phishing attachments, SharePoint and OneDrive links, SMB file shares, and drive-by downloads. No prior authentication to the target host is required.

No public proof-of-concept exploit has been published at the time of writing. Technical details are described in prose; readers can consult the Microsoft Security Update Guide for vendor-provided information.

Detection Methods for CVE-2026-97763

Indicators of Compromise

excel.exe spawning command interpreters such as cmd.exe, powershell.exe, or wscript.exe, especially shortly after opening an attachment.
Unexpected child processes writing to %APPDATA%, %TEMP%, or startup locations under the user profile.
Outbound network connections initiated by excel.exe to uncategorized or newly registered domains.
Excel crashes with heap corruption exceptions (c0000374, c0000005) correlated with the opening of specific documents.

Detection Strategies

Hunt for Office applications creating processes outside of normal productivity workflows, aligned with MITRE ATT&CK T1566.001 and T1204.002.
Alert on Excel loading unusual DLLs or scripting engines (vbscript.dll, jscript.dll) immediately after document open events.
Correlate email gateway telemetry for .xls, .xlsx, .xlsm, and .xlsb attachments with downstream endpoint execution events.

Monitoring Recommendations

Enable and forward Microsoft Office telemetry, Sysmon process creation (Event ID 1), and image load events to a central log pipeline.
Monitor for repeated Excel crashes across the fleet, which can indicate exploitation attempts or unreliable exploits in testing.
Review mail flow rules and web proxy logs for inbound spreadsheet files from external senders, particularly those with macros or embedded objects.

How to Mitigate CVE-2026-97763

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide across all Office installations.
Prioritize patching for users who routinely open spreadsheets from external sources, including finance, procurement, and executive assistants.
Verify that Office click-to-run and Microsoft Update channels are reporting successful patch deployment through configuration management tooling.

Patch Information

Microsoft has published guidance and update packages in the Microsoft Security Update Guide. Administrators should consult that advisory for build numbers and channel-specific KB identifiers before deployment.

Workarounds

Enforce Protected View for files originating from the internet and other untrusted locations through Group Policy or Intune.
Block or strip spreadsheet attachments from external senders at the mail gateway where business processes allow.
Disable legacy file formats such as XLS and XLSB using the Office File Block policy for high-risk user groups until patching is complete.
Require Mark-of-the-Web preservation on downloaded files so Office honors Protected View and Attack Surface Reduction rules.

bash

# Example: enable Office Attack Surface Reduction rule to block Office apps
# from creating child processes (run from an elevated PowerShell prompt)
Add-MpPreference -AttackSurfaceReductionRules_Ids `
  D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
  -AttackSurfaceReductionRules_Actions Enabled