CVE-2026-100209 – Microsoft Office Excel Remote Code Execution Vulnerability

Microsoft has addressed a critical Remote Code Execution (RCE) vulnerability in Microsoft Office Excel that could allow an unauthorized attacker to execute malicious code on a vulnerable system. The vulnerability, tracked as CVE-2026-100209, is caused by a Use-After-Free (UAF) memory management weakness. An attacker could exploit this vulnerability by convincing a user to open a specially crafted Office file. Successful exploitation could compromise the confidentiality, integrity, and availability of the affected system. Although Microsoft considers exploitation less likely, the vulnerability carries a high potential impact and requires security updates to protect affected environments.

CVSS SCORE: 8.8

SEVERITY: Critical

Threat

This vulnerability presents a significant security risk because it allows remote code execution through a malicious Microsoft Office document.

An attacker could distribute a specially crafted Excel file through phishing emails or other file-sharing methods and convince an employee to open it.

The attack requires no prior privileges, making it particularly concerning for organizations where employees regularly exchange spreadsheets with external parties.

Successful exploitation could allow attackers to execute unauthorized code, compromise sensitive information, modify business data, or disrupt normal system operations.

The vulnerability has a CVSS base score of 8.8, reflecting its potential for serious consequences.

Exploits

Microsoft has classified this vulnerability as Exploitation Less Likely.

• Publicly Disclosed: No
• Exploited in the Wild: No
• Exploit Code Maturity: Unproven
• Zero-Day Exploitation: No known exploitation is reported.
• Proof-of-Concept (PoC): No confirmed public PoC exploit is identified.

There is currently no confirmed evidence of active exploitation.

However, the vulnerability’s low attack complexity and lack of privilege requirements make it an important security concern.

Attackers would still need to convince a user to open a malicious Office document to trigger exploitation.

Technical Summary

CVE-2026-100209 is a critical Remote Code Execution vulnerability affecting Microsoft Office Excel.

The underlying issue is a Use-After-Free (CWE-416) memory management weakness.

A Use-After-Free vulnerability occurs when an application attempts to access memory that has already been released. Under certain conditions, this can cause unexpected application behavior and potentially allow an attacker to execute malicious code.

In this case, an attacker could exploit the vulnerability by creating a specially crafted Office document and persuading a target user to open it using a vulnerable version of Microsoft Excel.

The vulnerability has the following technical characteristics:

• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: Required
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

The attack does not require the attacker to possess existing privileges on the targeted system.

However, successful exploitation depends on user interaction, specifically opening a malicious Office document.

Microsoft also confirms that the Preview Pane is not an attack vector for this vulnerability.

Successful exploitation could result in unauthorized code execution, potentially allowing attackers to access sensitive information, alter files, or interfere with system availability.

Exploitability

The vulnerability affects the following Microsoft products:

• Microsoft 365 Apps for Enterprise – 32-bit Systems
• Microsoft 365 Apps for Enterprise – 64-bit Systems
• Microsoft Office LTSC 2024 – 32-bit Editions
• Microsoft Office LTSC 2024 – 64-bit Editions

An attacker could exploit the vulnerability by sending a specially crafted Office file and convincing a user to open it.

Exploitation requires no prior privileges but does require user interaction.

Microsoft rates exploitation as Less Likely, and no active exploitation has been reported.

The affected product editions are identified, but specific vulnerable build numbers and fixed build numbers are not specified.

Business Impact

This vulnerability creates a serious security concern for organizations that rely heavily on Microsoft Excel for daily business operations.

Excel spreadsheets are commonly exchanged between employees, customers, suppliers, and external partners, making malicious document delivery a realistic attack scenario.

Successful exploitation could result in:

• Sensitive Data Exposure: Attackers could gain unauthorized access to confidential business information, financial records, or customer data.
• Unauthorized Code Execution: Malicious code could execute on affected systems, potentially compromising the security of employee workstations.
• Business Data Manipulation: Attackers could modify or corrupt important business documents and operational information.
• Operational Disruption: A compromised system could experience service interruptions, application instability, or loss of availability.
• Financial and Reputational Damage: Security incidents involving sensitive information or disrupted business operations could lead to financial losses and reputational harm.

Organizations handling sensitive financial information or frequently exchanging Excel documents with external parties may face increased exposure.

“An ordinary spreadsheet could become an attacker’s entry point. Protecting Excel means protecting the business information employees depend on every day.”

Workaround

Microsoft has not identified any official mitigations or workarounds for this vulnerability.

If the security update cannot be deployed immediately, organizations should consider temporary precautionary measures:

• Avoid opening unexpected or suspicious Office attachments.
• Verify the sender before opening Excel files received from external sources.
• Use existing email security controls to help detect and block potentially malicious attachments.
• Limit the handling of untrusted Office documents until affected systems are updated.

These precautions may reduce exposure but do not eliminate the vulnerability.

Microsoft has identified an official fix, making security update deployment the recommended remediation.

Urgency

Critical Severity – Prioritize Patch Deployment

CVE-2026-100209 should be prioritized for remediation because it presents a critical Remote Code Execution risk with a CVSS score of 8.8.

The vulnerability requires no prior attacker privileges and has low attack complexity, increasing concern about potential exploitation through malicious Office documents.

Although Microsoft currently assesses exploitation as less likely and reports no known active exploitation, the potential impact remains significant.

Successful exploitation could compromise sensitive information, allow unauthorized code execution, and disrupt business operations.

Organizations should prioritize deploying the official Microsoft security update to affected Microsoft 365 Apps and Office LTSC 2024 installations.

Recommended Action: Deploy the security update through the organization’s established patch management process, prioritizing systems used to open Office documents received from external sources.