CVE-2026-44823: Microsoft Office Excel RCE Vulnerability

CVE-2026-44823 Overview

CVE-2026-44823 is an integer underflow vulnerability in Microsoft Office Excel that allows an unauthorized attacker to execute code locally. The flaw is classified under [CWE-197] Numeric Truncation Error, where arithmetic operations wrap around below the minimum representable value. Successful exploitation requires user interaction, typically by opening a crafted Excel document. The vulnerability impacts confidentiality, integrity, and availability on the affected host.

Critical Impact

An attacker who convinces a user to open a malicious spreadsheet can execute arbitrary code in the context of the current user, leading to full compromise of the local session.

Affected Products

• Microsoft Office Excel (refer to the Microsoft CVE-2026-44823 Update for the full list of impacted builds)

Discovery Timeline

• 2026-06-09 - CVE-2026-44823 published to the National Vulnerability Database
• 2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-44823

Vulnerability Analysis

The vulnerability resides in Excel's handling of numeric values during the parsing of spreadsheet structures. An integer underflow occurs when a calculation produces a value lower than the minimum representable integer, causing the result to wrap around to a large positive value. Excel then uses this corrupted value to size or index memory buffers, leading to memory corruption that an attacker can leverage for code execution.

The attack surface is local, but the actual delivery vector is typically a malicious document distributed through phishing, file shares, or web downloads. User interaction is required to open the document. Code runs with the privileges of the current user, so accounts running with administrative rights face broader system impact than standard users.

Root Cause

The root cause is improper validation of numeric input prior to arithmetic operations on a length, size, or offset field. When attacker-controlled values cause subtraction below zero, the wraparound produces an unexpectedly large unsigned value that bypasses subsequent bounds checks. This condition is consistent with [CWE-197] Numeric Truncation Error.

Attack Vector

The attacker crafts an Excel file containing malformed structures designed to trigger the underflow during parsing. The file is delivered to the victim through email attachments, collaboration platforms, or hosted downloads. When the victim opens the file in a vulnerable version of Excel, the underflow triggers memory corruption and shellcode execution. Preview pane interaction may also reach the vulnerable code path depending on configuration.

No public proof-of-concept exploit is currently listed, and the EPSS data places exploitation probability in the lower range. However, Office document parsing flaws are frequently weaponized after patch release through binary diffing.

Detection Methods for CVE-2026-44823

Indicators of Compromise

• Unexpected excel.exe child processes such as cmd.exe, powershell.exe, wscript.exe, or rundll32.exe
• Excel processes spawning network connections to untrusted external hosts shortly after opening an attachment
• Crash events or Windows Error Reporting entries for EXCEL.EXE referencing access violations during file parsing
• Spreadsheet files with malformed BIFF, OOXML, or embedded object structures arriving from external senders

Detection Strategies

• Hunt for anomalous parent-child process relationships originating from Office applications, focusing on script interpreters and LOLBins.
• Inspect Office telemetry for repeated application crashes tied to specific document hashes across multiple users.
• Correlate inbound email attachments containing .xls, .xlsx, .xlsm, or .xlsb files with subsequent endpoint anomalies on the recipient host.

Monitoring Recommendations

• Enable and forward Microsoft Defender Antimalware Scan Interface (AMSI) and Office telemetry to your SIEM for analysis.
• Monitor file write activity in user profile directories immediately following Excel document opens.
• Track outbound connections from Office processes and alert on traffic to newly registered or low-reputation domains.

How to Mitigate CVE-2026-44823

Immediate Actions Required

• Apply the security update referenced in the Microsoft CVE-2026-44823 Update advisory across all endpoints running Microsoft Office Excel.
• Block inbound Excel attachments from external senders at the email gateway until patches are deployed.
• Enforce Protected View and Office macro restrictions through Group Policy to reduce exposure from untrusted documents.

Patch Information

Microsoft has issued a security update through the Microsoft Security Response Center. Administrators should review the vendor advisory for affected product builds and deploy the corresponding cumulative update through Microsoft Update, WSUS, or Intune. Confirm patch installation by validating the Excel binary version against the build numbers listed in the MSRC guidance.

Workarounds

• Open untrusted spreadsheets only in Excel Protected View or Office for the Web, which sandbox document parsing.
• Configure Attack Surface Reduction (ASR) rules to block child process creation from Office applications.
• Restrict users from running Excel under administrative accounts to limit the blast radius of successful exploitation.

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

# Verify Excel build after patch deployment
(Get-Item "$env:ProgramFiles\Microsoft Office\root\Office16\EXCEL.EXE").VersionInfo.ProductVersion