CVE-2026-44820: Microsoft Excel RCE Vulnerability

CVE-2026-44820 Overview

CVE-2026-44820 is an integer underflow (wrap or wraparound) vulnerability in Microsoft Office Excel. The flaw allows an unauthorized attacker to execute code locally on a target system after a user opens a malicious file. The vulnerability is tracked under CWE-125 and stems from improper handling of numeric boundaries during file parsing.

Microsoft published advisory details through the Microsoft Vulnerability Advisory CVE-2026-44820. Exploitation requires user interaction, typically through opening a crafted Excel document delivered via email or web download.

Critical Impact

Successful exploitation grants attackers the ability to execute arbitrary code in the context of the current user, leading to full compromise of confidentiality, integrity, and availability on the affected host.

Affected Products

• Microsoft Office Excel (versions specified in the Microsoft advisory)
• Microsoft 365 Apps deployments containing the vulnerable Excel component
• Standalone Microsoft Office installations that include Excel

Discovery Timeline

• 2026-06-09 - CVE-2026-44820 published to the National Vulnerability Database
• 2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-44820

Vulnerability Analysis

The vulnerability resides in Microsoft Office Excel's parsing logic for spreadsheet content. An integer underflow occurs when an arithmetic operation produces a value below the minimum representable for its data type, wrapping around to a very large unsigned value. When Excel uses this wrapped value as a size or index, it triggers out-of-bounds memory access consistent with [CWE-125].

The Microsoft advisory classifies this issue as a local attack requiring user interaction. An attacker cannot exploit the flaw remotely without convincing a user to open a crafted file. Once opened, the malformed document drives Excel through the vulnerable code path and gains code execution in the user's security context.

Root Cause

The root cause is unsafe arithmetic on a length, offset, or counter field read from an untrusted Excel file. Subtracting an attacker-controlled value from a smaller field produces an underflow. The resulting unexpected value bypasses subsequent bounds checks, leading to memory corruption that an attacker shapes into control-flow hijacking.

Attack Vector

The attack vector is local with required user interaction. A typical scenario involves a phishing email delivering a weaponized .xlsx, .xls, or .xlsm file. The user opens the attachment, Excel parses the embedded structures, the integer underflow fires, and the attacker's payload executes with the privileges of the logged-on user. Protected View may reduce risk if the user does not enable editing.

No verified public proof-of-concept code is currently available for CVE-2026-44820. Technical specifics are documented in the Microsoft Vulnerability Advisory CVE-2026-44820.

Detection Methods for CVE-2026-44820

Indicators of Compromise

• Unexpected child processes spawned from excel.exe, especially cmd.exe, powershell.exe, wscript.exe, mshta.exe, or rundll32.exe
• Excel processes writing executable content to %TEMP%, %APPDATA%, or %PROGRAMDATA%
• Outbound network connections initiated directly by excel.exe to uncategorized or newly registered domains
• Crash events in excel.exe with access violation exceptions correlating with recently opened email attachments

Detection Strategies

• Hunt for Office applications spawning script interpreters or living-off-the-land binaries, mapped to MITRE ATT&CK techniques T1566.001 and T1059
• Inspect email gateways and sandbox detonations for Excel documents containing malformed binary structures or anomalous record sizes
• Correlate Excel crash telemetry with subsequent process creation or persistence events on the same host

Monitoring Recommendations

• Enable Microsoft Defender Attack Surface Reduction rules that block child process creation from Office applications
• Forward Sysmon Event IDs 1, 7, and 11 from endpoints running Office to a centralized analytics platform
• Track AMSI-instrumented script execution originating from Office host processes

How to Mitigate CVE-2026-44820

Immediate Actions Required

• Apply the Microsoft security update for CVE-2026-44820 to all systems running Microsoft Office Excel as soon as it is available through Microsoft Update channels
• Enforce Protected View and Office macro restrictions for files originating from the internet and email attachments
• Block or quarantine inbound Excel attachments from untrusted senders at the email gateway pending patch deployment

Patch Information

Microsoft has published remediation guidance in the Microsoft Vulnerability Advisory CVE-2026-44820. Administrators should deploy updates via Windows Update, Microsoft Update, WSUS, Intune, or Microsoft 365 Apps update channels depending on the Office distribution model in use. Verify build numbers post-deployment to confirm the patched version is active.

Workarounds

• Configure Office to open files from the internet in Protected View and prevent users from clicking Enable Editing for unsolicited documents
• Deploy Attack Surface Reduction rule D4F940AB-401B-4EFC-AADC-AD5F3C50688A to block Office applications from creating child processes
• Restrict Excel file types at the email gateway and require sandbox detonation before delivery to user mailboxes
• Apply the principle of least privilege so that exploitation does not yield administrative access on the endpoint

# Enable ASR rule to block Office child processes via PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A `
                 -AttackSurfaceReductionRules_Actions Enabled