# CVE-2026-100209: Microsoft Office Excel use-after-free code execution

Source: https://junglewise.ai/threats/cve-2026-100209-use-after-free-in-microsoft-office-excel-allows-an-unauthorized  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-13. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-100209 · Severity: high · CVSS 8.8 · Published 2026-10-13

Microsoft Office Excel contains a use-after-free vulnerability that could allow an attacker to execute arbitrary code when a user opens a specially crafted file. An attacker could exploit this remotely to gain full control of a system, potentially leading to data theft, malware installation, or disruption of business operations.

Technologies: [Microsoft 365 Apps](https://junglewise.ai/threats/technologies/365-apps), [Microsoft 365](https://junglewise.ai/threats/technologies/microsoft-365), [Microsoft Office 2021](https://junglewise.ai/threats/technologies/office-2021), [Microsoft Office 2024](https://junglewise.ai/threats/technologies/office-2024), [Microsoft Office Excel](https://junglewise.ai/threats/technologies/office-excel). Vendors: [Microsoft](https://junglewise.ai/threats/vendors/microsoft).

## Quick answers

**Is CVE-2026-100209 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-100209 being exploited in the wild.

**How severe is CVE-2026-100209?** High severity, with a CVSS score of 8.8, published 13 October 2026.

**Which products and versions are affected?** Microsoft Office Excel.

**Is there a fix for CVE-2026-100209?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-100209 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-100209?** Check whether you run Microsoft 365 Apps, Microsoft 365, Microsoft Office 2021 or one of the others listed. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A use-after-free vulnerability exists in Excel's memory handling, allowing an attacker to execute code by crafting a malicious spreadsheet file. The vulnerability requires user interaction (opening a file) and can be delivered over a network. Exploitation grants the attacker arbitrary code execution in the context of the user running Excel.

## Affected products

- Microsoft Office Excel

## Timeline

- 2026-10-13: disclosed

## References

- [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-100209](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-100209)

## Related threats

- [Microsoft Office Excel unsafe deserialization in file parsing](https://junglewise.ai/threats/cve-2026-124229-deserialization-of-untrusted-data-in-microsoft-office-excel)
- [Microsoft Office Excel use-after-free remote code execution](https://junglewise.ai/threats/cve-2026-124187-use-after-free-in-microsoft-office-excel-allows-an-unauthorized)
- [Microsoft Office Excel buffer over-read information disclosure](https://junglewise.ai/threats/cve-2026-124183-buffer-over-read-in-microsoft-office-excel-allows-an)
- [Microsoft Office Excel out-of-bounds read information disclosure](https://junglewise.ai/threats/cve-2026-124179-out-of-bounds-read-in-microsoft-office-excel-allows-an)
- [Microsoft Office Excel out-of-bounds read](https://junglewise.ai/threats/cve-2026-124178-out-of-bounds-read-in-microsoft-office-excel-allows-an)