CVE-2026-97773: Microsoft Excel Buffer Overflow Vulnerability

CVE-2026-97773 Overview

CVE-2026-97773 is a heap-based buffer overflow vulnerability in Microsoft Office Excel that enables remote code execution. An unauthorized attacker can execute arbitrary code over a network by delivering a crafted Excel file to a target user. The flaw is categorized under CWE-122, heap-based buffer overflow. Exploitation requires user interaction, typically opening a malicious spreadsheet. Microsoft published the advisory through the Microsoft Security Response Center.

Critical Impact
Successful exploitation grants attackers code execution in the context of the current user, which can lead to full system compromise when the user holds administrative privileges.

Affected Products

Microsoft Office Excel (see the Microsoft Security Update for CVE-2026-97773 for affected builds)
Microsoft 365 Apps components that embed the Excel rendering engine
Office standalone versions supported by Microsoft at the time of the advisory

Discovery Timeline

2026-10-13 - CVE CVE-2026-97773 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-97773

Vulnerability Analysis

The vulnerability resides in how Excel parses structured content inside spreadsheet files. A heap-based buffer overflow occurs when Excel writes attacker-controlled data beyond the bounds of a heap-allocated buffer. Overwriting adjacent heap metadata or function pointers allows an attacker to redirect execution to shellcode. The attack vector is network-based because malicious documents are delivered through email, web downloads, or shared storage. User interaction is required, meaning the victim must open the crafted file.

Root Cause

The root cause is improper validation of input size during the processing of a specific record or object within an Excel document. When the length of the input exceeds the allocated buffer, Excel writes outside the intended heap region. This behavior aligns with CWE-122 and is a common class of memory-safety flaw in legacy document parsers.

Attack Vector

An attacker crafts a malicious Excel workbook that triggers the overflow during parsing. The file is delivered through phishing emails, malicious links, or compromised file shares. When the victim opens the document, Excel processes the malformed structure, overflows the heap buffer, and transfers control to attacker-supplied code. The Preview Pane may also serve as an attack surface for certain Office vulnerabilities, though specific vectors should be confirmed in the Microsoft advisory.

No public proof-of-concept code is available at the time of writing. The vulnerability is described in prose because no verified exploit samples have been published.

Detection Methods for CVE-2026-97773

Indicators of Compromise

Unexpected child processes spawned by excel.exe, such as cmd.exe, powershell.exe, rundll32.exe, or wscript.exe
Excel processes making outbound network connections to unknown hosts shortly after document open
Crash events or Windows Error Reporting entries referencing EXCEL.EXE with heap corruption signatures
Suspicious .xls, .xlsx, .xlsb, or .xlsm files arriving via email with macros or embedded objects

Detection Strategies

Hunt for process-ancestry chains where Microsoft Office applications launch scripting or living-off-the-land binaries
Correlate email gateway telemetry with endpoint document-open events to flag high-risk attachments
Monitor for file writes by EXCEL.EXE into user profile paths commonly abused for persistence, such as %AppData% and %Temp%

Monitoring Recommendations

Enable attack surface reduction (ASR) rules that block Office applications from creating child processes
Ingest Microsoft Defender, Sysmon, and email security telemetry into a centralized SIEM for cross-source correlation
Alert on Excel crashes paired with subsequent anomalous network or process activity on the same host

How to Mitigate CVE-2026-97773

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update for CVE-2026-97773 to all affected systems
Prioritize patching for endpoints used by high-value or privileged users
Instruct users to avoid opening unsolicited Excel attachments and to report suspicious files

Patch Information

Microsoft has released an official security update for CVE-2026-97773. Administrators should deploy the patch through Windows Update, Microsoft Update, Microsoft Intune, Configuration Manager, or Microsoft 365 Apps update channels. Refer to the MSRC advisory for exact build numbers and update packages.

Workarounds

Open untrusted Excel files in Protected View or Application Guard to isolate parser execution
Disable the Outlook Preview Pane to reduce the risk of automatic document rendering
Block inbound Excel attachments at the email gateway from external or untrusted senders until patches are deployed
Enforce Microsoft Defender ASR rule to block Office child-process creation

bash

# Enable ASR rule: Block Office applications from creating child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled