Microsoft has released emergency out-of-band security updates to patch a high-severity Microsoft Office zero-day vulnerability exploited in attacks.

The security feature bypass vulnerability, tracked as CVE-2026-21509, affects multiple Office versions, including Microsoft Office 2016, Microsoft Office 2019, Microsoft Office LTSC 2021, Microsoft Office LTSC 2024, and Microsoft 365 Apps for Enterprise (the company's cloud-based subscription service).

However, as noted in today's advisory, security updates for Microsoft Office 2016 and 2019 are not yet available and will be released as soon as possible.

While the preview pane is not an attack vector, unauthenticated local attackers can still successfully exploit the vulnerability through low-complexity attacks that require user interaction.

"Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally. An attacker must send a user a malicious Office file and convince them to open it," Microsoft explained.

"This update addresses a vulnerability that bypasses OLE mitigations in Microsoft 365 and Microsoft Office which protect users from vulnerable COM/OLE controls."

"Customers on Office 2021 and later will be automatically protected via a service-side change, but will be required to restart their Office applications for this to take effect," it added.

Although Office 2016 and 2019 aren't immediately patched against attacks, Microsoft has provided confusing mitigation measures that could "reduce the severity of exploitation."

We have attempted to clear this up with our instructions below:

1. Close all Microsoft Office applications.
2. Create a backup of the Windows Registry, as incorrectly editing it can cause issues with the operating system.
3. Open the Windows Registry Editor (regedit.exe) by clicking on the Start menu and typing regedit, and then pressing Enter when it appears in the search results.
4. When open, use the address bar at the top to see if one of the following Registry keys exists:

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\16.0\Common\COM Compatibility\ (for 64-bit Office, or 32-bit Office on 32-bit Windows)
   
   HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Office\16.0\Common\COM Compatibility\ (for 32-bit Office on 64-bit Windows)
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Microsoft\Office\16.0\Common\COM Compatibility\
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\WOW6432Node\Microsoft\Office\16.0\Common\COM Compatibility\

   If one of the above keys does not exist, create a new "COM Compatibility" key under this Registry path by right-clicking on Common and selecting New -> Key.

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\16.0\Common\

5. Now right-click on the existing or newly created COM Compatibility key and select New -> Key and name it {EAB22AC3-30C1-11CF-A7EB-0000C05BAE0B}.
6. When the new {EAB22AC3-30C1-11CF-A7EB-0000C05BAE0B} is created, right-click on it, select New -> DWORD (32-bit) Value. Name the new value Compatibility Flags.
7. When the Compatibility Flags value is created, double-click on it, make sure the Base option is set to Hexadecimal, and enter 400 in the Value data field.

After performing these steps, the flaw will be mitigated when you next launch an Office application.

Microsoft has not shared who discovered the vulnerability or any details on how it is exploited, and a spokesperson was not immediately available for comment when contacted by BleepingComputer earlier today.

Earlier this month, as part of the January 2026 Patch Tuesday, Microsoft issued security updates for 114 flaws, including one actively exploited and two publicly disclosed zero-day bugs.

The other actively exploited zero-day patched this month is an information disclosure flaw in the Desktop Window Manager, tagged by Microsoft as "important severity," that can let attackers to read memory addresses associated with the remote ALPC port.

Last week, Microsoft also released multiple out-of-band Windows updates to fix shutdown and Cloud PC bugs triggered by the January Patch Tuesday updates, as well as another set of emergency updates to address an issue causing the classic Outlook email client to freeze or hang.