Cyber: Microsoft Patches Actively Exploited Office Zero-day Vulnerability

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:

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.

Source: BleepingComputer