⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 8.4

How to Fix CVE-2026-40366: Use-After-Free in Microsoft 365 Apps for Enterprise

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 8.4 - High
Actively exploited?Not currently listed in CISA KEV
Affected16.0.1 < https://aka.ms/OfficeSecurityReleases, 19.0.0 < https://aka.ms/OfficeSecurityReleases, 16.0.1 < https://aka.ms/OfficeSecurityReleases, 16.0.0 < https://aka.ms/OfficeSecurityReleases, 16.0.1 < 16.109.26051019, 16.0.0 < 16.109.26051019, and others
Fixed inSee vendor advisory
Type (CWE)CWE-416: Use After Free

What is CVE-2026-40366?

CVE-2026-40366 is an use-after-free bug in Microsoft 365 Apps for Enterprise. A reference to freed memory is dereferenced later in the program, allowing an attacker who controls the reallocated content to hijack execution. Vendor description: Use after free in Microsoft Office Word allows an unauthorized attacker to execute code locally.

Why this CVE matters

Use-after-free vulnerabilities in a network or media-parsing path tend to draw immediate exploit development effort. The bug class is well understood, and public toolkits exist that adapt quickly to newly disclosed cases.

For deployments of Microsoft 365 Apps for Enterprise that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

On Windows, check the product's installed version via Settings - Apps - Installed apps, or run Get-Package from PowerShell to enumerate installed versions.

How to fix CVE-2026-40366

  1. Read the vendor advisory in full: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40366
  2. Upgrade Microsoft 365 Apps for Enterprise to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

<!-- enrich-agent-8 -->

Apply the Microsoft security update for Microsoft 365 apps for enterprise

Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40366


# Confirm the patch is missing on this host.
Get-Hotfix -ErrorAction SilentlyContinue | Sort-Object InstalledOn -Descending |
  Select-Object -First 10

# Install the latest cumulative + security rollup that ships the fix for Microsoft 365 Apps for Enterprise.
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Import-Module PSWindowsUpdate
Get-WindowsUpdate -AcceptAll -Install -AutoReboot

# After reboot, verify the patched build is in place (https://aka.ms/OfficeSecurityReleases).
Get-ComputerInfo -Property WindowsVersion, OsBuildNumber, OsHardwareAbstractionLayer


# Inventory missing patches across many Windows hosts via Ansible (winrm).
ansible windows -m win_updates -a "category_names=SecurityUpdates state=installed"

Verify the fix landed


# Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40366
# 1. Confirm the running version matches the fixed-in version listed above.

# 2. Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
#    The scanner should no longer flag this CVE on the patched target.

# 3. Inspect recent service / kernel logs for crash-loops or rollback events.
journalctl -u <service> --since "10 minutes ago"
dmesg --since "10 minutes ago"

<!-- enrich-agent-8 -->

If you cannot patch immediately

Block network reachability to the vulnerable service from untrusted networks and apply the patched build. Memory-corruption bugs cannot be reliably mitigated at the network layer; the patch is the fix.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for repeated service restarts, crash logs from the affected daemon, and core files generated around the time of any anomalous traffic. A memory-corruption flaw used for exploitation often leaves a trail of failed attempts before the successful one.

Frequently asked questions

Is CVE-2026-40366 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-40366?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for Microsoft 365 Apps for Enterprise run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*