Excel Use-After-Free CVE-2026-100209 Rated High Severity, Risks Code Execution on Older Office Builds

Microsoft Office Excel contains a use-after-free vulnerability that permits an unauthorized attacker to execute code over a network. This flaw, identified as CVE-2026-100209, is rated high severity with a CVSS score of 8.8 by the National Vulnerability Database. As BlueTeam Insider reported on 2026-10-12 regarding NetEase Cloud Music, memory corruption flaws can lead to arbitrary code execution, but this Excel issue is distinct in its vector.

Root cause

The underlying weakness is classified as CWE-416, a use-after-free error. This occurs when a program continues to use a pointer after the memory it references has been freed. The National Vulnerability Database description confirms this specific mechanism enables remote code execution capabilities for unauthorized users. The source material does not specify further technical details regarding the internal memory handling.

Attack path

An unauthorized attacker can exploit this vulnerability over a network. The source material does not specify the exact trigger mechanism beyond the network vector. This implies that the exploit can be delivered through network services or protocols that handle Excel files, rather than requiring local physical access. Teams should assume that any network-delivered Excel content could potentially trigger this memory corruption.

Affected versions

The following Microsoft products are affected before the specified versions:

• Microsoft 365 Apps for Enterprise before the Office Security Releases publication.
• Microsoft Office 365 for Mac before the publication date.
• Microsoft Office LTSC 2024 before build 16.0.17932.21056.
• Microsoft Office LTSC for Mac 2021 before the publication date.
• Microsoft Office LTSC for Mac 2024 before the publication date.

Mitigation

• Apply the latest Office Security Releases to Microsoft 365 Apps for Enterprise.
• Update Microsoft Office LTSC 2024 to build 16.0.17932.21056 or later.
• Ensure Mac-based Office 365 and LTSC versions are updated to the latest available patches.

Detection notes

Teams usually monitor for anomalous Excel processes spawning child processes, such as PowerShell or command shells. Check whether Excel instances are accessing network resources unexpectedly, which may indicate exploitation attempts. Review memory access logs for signs of heap corruption or unusual pointer dereferencing. Isolate affected endpoints immediately upon detection to prevent lateral movement. Validate that endpoint detection and response tools are tuned to catch suspicious process injection patterns associated with memory corruption exploits.

Background: Process injection

Process injection fails when you rely solely on signature detection. You must enforce strict memory protection settings, restrict process creation rights, and monitor for abnormal parent-child relationships. These structural controls prevent attackers from hiding their malicious code inside legitimate system processes that bypass standard security checks.

Read the full guide: Process Injection Mistakes That Leave Your Systems Open

What to do and how to stay safe: Microsoft

• Verify your Excel version against the affected list provided in the advisory.
• Apply pending Office updates to reach the secure build numbers mentioned.
• Monitor network traffic for unusual Excel-related outbound connections.
• Once the vendor provides an update, prioritize deployment to internet-facing workstations.

Step-by-step guide: Unpatched Software Myths: Why Common Beliefs Fail to Protect You

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

Frequently asked questions

What is the severity of CVE-2026-100209?

The National Vulnerability Database rates the severity as high with a CVSS score of 8.8.

Which Excel versions are affected?

Microsoft 365 Apps for Enterprise, Office 365 for Mac, and LTSC versions before specific builds are affected.

Does this require user interaction?

The source material states an unauthorized attacker can execute code over a network, but does not specify user interaction requirements.