High-Severity Use-After-Free Flaw in Microsoft 365 Apps for Enterprise Tracked as CVE-2026-100209
A newly published high vulnerability in Microsoft 365 Apps for Enterprise carries a CVSS score of 8.8. Here is what is affected and how to respond.
Key points
- CVE-2026-100209 affects Microsoft 365 Apps for Enterprise.
- CVSS 8.8 (High).
- No exploitation confirmed yet.
A high use-after-free tracked as CVE-2026-100209 has been published in the National Vulnerability Database for Microsoft 365 Apps for Enterprise, with a CVSS base score of 8.8.
At a glance
- CVE
- CVE-2026-100209
- Product
- Microsoft 365 Apps for Enterprise
- CVSS
- 8.8 High
- Weakness
- use-after-free
- Exploited
- No known exploitation
What is the vulnerability?
The NVD entry states: Use after free in Microsoft Office Excel allows an unauthorized attacker to execute code over a network.
Affected versions
| Vendor | Product | Versions |
|---|---|---|
| Microsoft | Microsoft 365 Apps for Enterprise | before https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Office 365 for Mac | before publication |
| Microsoft | Microsoft Office LTSC 2024 | before 16.0.17932.21056 |
| Microsoft | Microsoft Office LTSC for Mac 2021 | before publication |
| Microsoft | Microsoft Office LTSC for Mac 2024 | before publication |
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C.
Is it being exploited?
There is no public confirmation of exploitation at the time of publication. It does not appear in CISA's Known Exploited Vulnerabilities catalog. We will update this report if that changes.
What should you do?
- Identify whether the affected product and versions run in your environment.
- Apply the vendor update listed in the advisory as soon as it is available.
- Limit network exposure of the affected service until it is patched.
- Monitor the vendor advisory for updated guidance or exploitation reports.
What to do and how to stay safe: CVE-2026-100209
- Vulnerability severity scores ignore your specific architecture and context.
- Exploitability depends on network exposure and user behavior, not just code flaws.
- Prioritization requires continuous asset inventory and threat intelligence alignment.
Risk-based vulnerability management focuses your limited resources on the flaws that actually threaten your business. Start by mapping your critical assets and checking for active exploits to prioritize your backlog.
Step-by-step guide: Risk-Based Vulnerability Management: Prioritize Fixes by Actual Danger
General security guidance from the Threat Times newsroom. It is not confirmed advice from the organisations named in this story.
Frequently asked questions
What is CVE-2026-100209?
CVE-2026-100209 is a use-after-free vulnerability in Microsoft 365 Apps for Enterprise.
How severe is CVE-2026-100209?
It has a CVSS score of 8.8 High.
Is CVE-2026-100209 being exploited?
There is no public confirmation of exploitation at the time of publication.
