CVE-2026-55029 in Microsoft 365 Apps
Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
Quick answer
Microsoft 365 Apps should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- Microsoft 365 Apps
- Excel 2016
- Microsoft 365
- Office 2019
- Office 2021
- Office 2024
- Office Online Server before 16.0.10417.20175
Fixed versions
- Office Online Server 16.0.10417.20175 or later
- Apply the Microsoft security update for CVE-2026-55029
How to fix it
CVE-2026-55029 is a remote code execution issue in Microsoft 365 Apps. Update from Microsoft 365 Apps to Office Online Server 16.0.10417.20175 or later or newer. Keep it simple: upgrade first, then test the normal user flow.
- Find every place where Microsoft 365 Apps is installed or used.
- Upgrade Microsoft 365 Apps to Office Online Server 16.0.10417.20175 or later or a newer fixed release from the vendor.
- If you cannot upgrade right away, limit access to the affected feature until the update is done.
- Restart or redeploy the service if the product needs it after the update.
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Verify the fix
- Confirm the installed version is Office Online Server 16.0.10417.20175 or later or newer.
- Run a normal login, admin, and user flow to make sure the site still works.
- Check logs for new errors after the update.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-55029?
Microsoft 365 Apps versions listed as affected should be reviewed: Microsoft 365 Apps, Excel 2016, Microsoft 365, Office 2019, Office 2021, Office 2024, Office Online Server before 16.0.10417.20175.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.