CVE-2026-45469: Microsoft Excel RCE Vulnerability

CVE-2026-45469 Overview

CVE-2026-45469 is a heap-based memory corruption vulnerability in Microsoft Office Excel. The flaw stems from an integer underflow (wrap or wraparound) condition, classified under [CWE-122] as a heap-based buffer overflow. An unauthorized attacker can exploit this weakness to execute arbitrary code on the targeted system. Exploitation requires user interaction, typically opening a crafted Excel document. Microsoft published the security advisory on June 9, 2026.

Critical Impact
Successful exploitation allows attackers to execute arbitrary code in the context of the current user, leading to full compromise of confidentiality, integrity, and availability on the local system.

Affected Products

Microsoft Office Excel
Microsoft Office (Excel component)
Refer to the Microsoft Security Update Guide for the complete list of impacted builds

Discovery Timeline

2026-06-09 - CVE-2026-45469 published to NVD
2026-06-09 - Microsoft releases security advisory and patch
2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-45469

Vulnerability Analysis

The vulnerability is an integer underflow in Microsoft Office Excel's file parsing logic. When Excel processes a malformed spreadsheet, a length or size value undergoes arithmetic that wraps below zero. The wrapped value is then used to size or index into a heap allocation. This produces a heap-based buffer overflow [CWE-122], allowing the attacker to corrupt adjacent memory structures.

Attackers craft a malicious .xlsx, .xls, or related Excel document containing manipulated record sizes. When the victim opens the file, Excel's parser performs an unchecked subtraction that underflows. The resulting oversized or undersized buffer operation overwrites heap metadata or function pointers. Controlled corruption of these structures yields arbitrary code execution in the user's security context.

The attack vector is local and requires user interaction. Exploitation does not require any privileges on the target system, making delivery via phishing email attachments or hosted documents a viable path.

Root Cause

The root cause is missing validation of an integer value used in buffer size calculations. Excel decrements or subtracts a length field without verifying that the operand is large enough. When the subtraction underflows, the value wraps to a large positive integer, bypassing subsequent size checks during heap allocation or copy operations.

Attack Vector

An attacker delivers a weaponized Excel document to the target through email, messaging platforms, or web download. The victim opens the file, and Excel's parser triggers the underflow during record processing. The resulting heap corruption is leveraged to redirect execution to attacker-controlled shellcode. Code runs with the privileges of the user opening the document.

No verified proof-of-concept code is publicly available at this time. See the Microsoft Security Update Guide for vendor-confirmed technical details.

Detection Methods for CVE-2026-45469

Indicators of Compromise

Excel processes (EXCEL.EXE) spawning unexpected child processes such as cmd.exe, powershell.exe, rundll32.exe, or wscript.exe
Crashes in EXCEL.EXE followed by anomalous outbound network connections
Office documents arriving from external senders with embedded macros, OLE objects, or unusually structured records
Unexpected writes to %APPDATA%, %TEMP%, or Office startup folders following document open events

Detection Strategies

Inspect Office telemetry for Excel process anomalies, including unsigned module loads and abnormal memory allocations during document open
Apply YARA rules against inbound documents to flag malformed record sizes or known exploitation patterns
Correlate Excel crash events (Windows Error Reporting) with subsequent process creation or network activity
Monitor for Office child-process creation chains, a common post-exploitation indicator

Monitoring Recommendations

Enable Microsoft Defender Attack Surface Reduction (ASR) rule Block all Office applications from creating child processes and forward audit events to a central log
Collect Sysmon event IDs 1 (process create), 7 (image load), and 11 (file create) from endpoints running Office
Ingest Office telemetry and endpoint logs into a centralized analytics platform for hunting on Excel-initiated suspicious behavior
Alert on Excel processes loading unsigned DLLs or executing from user-writable directories

How to Mitigate CVE-2026-45469

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide to all systems running Microsoft Office Excel
Prioritize patching endpoints used by high-risk users such as executives, finance, and HR personnel
Enforce Protected View for documents originating from the internet or email attachments
Block inbound Excel attachments at the email gateway pending patch deployment

Patch Information

Microsoft has released a security update addressing CVE-2026-45469. Administrators should deploy the update through Microsoft Update, Windows Server Update Services (WSUS), Microsoft Intune, or Microsoft Configuration Manager. Verify patch application by checking the build number of EXCEL.EXE against the version listed in the vendor advisory.

Workarounds

Enable Microsoft Defender ASR rules to block Office applications from creating child processes and injecting code into other processes
Configure Office File Block policy to prevent opening legacy Excel binary formats (.xls, .xlsb) from untrusted locations
Disable macros from the internet via Group Policy and require Trusted Locations for macro-enabled files
Train users to avoid opening unsolicited Excel attachments and to report suspicious documents to the security team

bash

# Configuration example: Enable ASR rule to block Office child processes
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled

# Verify the rule is applied
Get-MpPreference | Select-Object -ExpandProperty AttackSurfaceReductionRules_Ids