CVSS 8.8 Excel Flaw CVE-2026-100209 Lets Attackers Run Code Remotely

Microsoft has identified a high-severity security flaw in Excel that permits remote code execution. This vulnerability, tracked as CVE-2026-100209, stems from a use-after-free error within the application. According to the National Vulnerability Database, an unauthorized attacker can exploit this issue to run arbitrary code on a victim's machine over a network connection. The flaw poses a significant risk to users who open malicious Excel files.

The risk

The Common Vulnerability Scoring System rates CVE-2026-100209 at 8.8, which falls into the high severity band. This score reflects the potential for remote exploitation without prior authentication. The underlying weakness is classified as CWE-416, a use-after-free error. In this scenario, the software continues to reference memory that has already been freed. This can lead to unexpected behavior, crashes, or the execution of attacker-controlled code. Microsoft’s official advisory confirms that the vulnerability allows an unauthorized attacker to execute code over a network.

Who is exposed

Several versions of Microsoft Office are affected by this vulnerability. According to the NVD record, Microsoft 365 Apps for Enterprise is vulnerable in builds prior to the latest security releases. Users of Microsoft Office 365 for Mac are also at risk until a fix is published. Additionally, Microsoft Office LTSC 2024 is affected in versions before 16.0.17932.21056. The NVD also lists Microsoft Office LTSC for Mac 2021 and 2024 as vulnerable before their respective publication dates. These older builds remain susceptible until updated.

Cost and compliance

The source material does not provide specific data on financial costs or compliance impacts. There is no mention of regulatory fines or direct economic losses in the advisory. The NVD record focuses solely on the technical classification and affected products. No details regarding insurance implications or legal liabilities are included in the provided texts. Organizations must evaluate their own risk tolerance and compliance frameworks. They should assess how this high-severity flaw aligns with their internal security policies.

What should you do?

Managers should verify which versions of Excel are currently deployed in their environment. They need to check if their systems are running builds older than the specified safe versions. Teams should prioritize updating affected machines to the latest available releases. Monitoring network traffic for unusual activity can help detect potential exploitation attempts. Ensuring that users do not open untrusted Excel files is also a prudent immediate measure.

The background in plain language

Use-after-free errors occur when software accesses memory that has already been released. This is like trying to read a book after it has been shredded. The data in that memory space may have changed or been overwritten by other processes. Attackers can manipulate this leftover memory to trick the program into running malicious instructions. Understanding this concept helps explain why a simple file open action can lead to full system compromise. It highlights the importance of keeping software updated to patch these low-level memory management flaws.

Background: Remote code execution vulnerabilities

Small businesses face high remote code execution risks due to limited patching resources and complex dependencies. You can mitigate this by enforcing strict input validation, maintaining an accurate IT asset inventory, and requiring vendors to demonstrate automated security regression testing for all updates.

Read the full guide: Remote Code Execution Risks and Protection for Small Businesses

What to do and how to stay safe: Microsoft

• Audit your network to identify all devices running vulnerable versions of Excel.
• Apply the latest security updates from Microsoft to affected systems immediately.
• Educate users to avoid opening Excel files from unknown or suspicious sources.
• Monitor system logs for signs of unauthorized code execution or unusual behavior.

Step-by-step guide: Zero-Day Vulnerabilities Meaning: How Unpatched Flaws Bypass Your Defenses

General security guidance from the Vector Tribune newsroom. It is not confirmed advice from the organisations named in this story.

Frequently asked questions

What is the CVSS score for CVE-2026-100209?

The CVSS score is 8.8, which is rated as high severity.

Does this vulnerability affect Mac versions of Office?

Yes, it affects Microsoft Office 365 for Mac and Office LTSC for Mac 2021 and 2024.

Is there a confirmed fix for all affected versions?

Microsoft 365 Apps for Enterprise and Office LTSC 2024 have specific version thresholds. Mac versions are listed as vulnerable before publication, implying updates are pending or just released.