Advisory · CVE-2026-100212
Use after free in Microsoft Office Excel can run code from a crafted file
Microsoft Office Excel has a use after free flaw a remote attacker can use to execute code when a user opens a crafted file. Microsoft rates it CVSS 3.1 8.8.
- Vendor
- Microsoft
- Product
- Microsoft Office Excel
- Identifier / CWE
- CVE-2026-100212
CWE-416 - Action timing
- Immediate
Explain it like I’m five
A second Excel use-after-free in the same cycle: same story, different crack in the file parser.
- 01Attacker engages the component
A remote or authorized attacker sends crafted content or requests to Microsoft Office Excel.
- 02Weakness triggers
The component mishandles the input and the flaw takes effect.
- 03Impact lands
The attacker gains the execution, elevation, or access the flaw allows.
What happened
Microsoft disclosed CVE-2026-100212 in its October 2026 update guide: a use after free weakness (CWE-416) in Microsoft Office Excel that lets a remote attacker execute code through a crafted file. Microsoft’s affected data covers the Microsoft 365 Apps and Office LTSC release lines.
Microsoft lists the affected releases and their patched build thresholds in its update guide. Apply the October 2026 security updates for your release rather than relying on a version guess here.
What to do
- Inventory every Microsoft Office Excel deployment, including test, training, and individually managed instances.
- Apply the vendor fix or upgrade exactly as directed in the linked vendor bulletin or advisory.
- Restrict network access to Microsoft Office Excel while remediation is underway, especially for internet-facing instances.
- Review application and server logs for Excel crashes across multiple recipients of the same workbook.
- Check for accounts, content, or configuration created outside normal change processes, and rotate exposed credentials where access cannot be ruled out.
- Verify the deployed version or patch level after remediation.
Management note
Crafted Excel files are a delivery staple. Push the Office updates with the OS set and treat crash clusters as a hunting lead.