Microsoft issued an emergency patch for a high-severity zero-day vulnerability in Office that allows attackers to bypass document security checks and is being exploited in the wild via malicious files.
Microsoft pushed the emergency patch for the zero‑day, tracked as CVE-2026-21509, and classified it as a “Microsoft Office Security Feature Bypass Vulnerability” with a CVSS score of 7.8 out of 10.
The flaw allows attackers to bypass Object Linking and Embedding (OLE) mitigations that are designed to block unsafe COM/OLE controls inside Office documents. This means a malicious attachment could infect a PC despite built-in protections.
In a real-life scenario, an attacker creates a fake Word, Excel, or PowerPoint file containing hidden “mini‑programs” or special objects. They can run code and do other things on the affected computer. Normally, Office has safety checks that would block those mini-programs because they’re risky.
However, the vulnerability allows the attacker to tweak the file’s structure and hidden information in a way that tricks Office into thinking the dangerous mini‑program inside the document is harmless. As a result, Office skips the usual security checks and allows the hidden code to run.
As code to test the bypass is publicly available, increasing the risk of exploitation, users are under urgent advice to apply the patch.
What you need to do depends on which version of Office you’re using.
The affected products include Microsoft Office 2016, 2019, LTSC 2021, LTSC 2024, and Microsoft 365 Apps (both 32‑bit and 64‑bit).
Office 2021 and later are protected via a server‑side change once Office is restarted. To apply it, close all Office apps and restart them.
Office 2016 and 2019 require a manual update. Run Windows Update with the option to update other Microsoft products turned on.
If you’re running build 16.0.10417.20095 or higher, no action is required. You can check your build number by opening any Office app, going to your account page, and selecting About for whichever application you have open. Make sure the build number at the top reads 16.0.10417.20095 or higher.
What always helps:
We don’t just report on threats—we remove them
Cybersecurity risks should never spread beyond a headline. Keep threats off your devices by downloading Malwarebytes today.
Microsoft issued an emergency patch for a high-severity zero-day vulnerability in Office that allows attackers to bypass document security checks and is being exploited in the wild via malicious files.
Microsoft pushed the emergency patch for the zero‑day, tracked as CVE-2026-21509, and classified it as a “Microsoft Office Security Feature Bypass Vulnerability” with a CVSS score of 7.8 out of 10.
The flaw allows attackers to bypass Object Linking and Embedding (OLE) mitigations that are designed to block unsafe COM/OLE controls inside Office documents. This means a malicious attachment could infect a PC despite built-in protections.
In a real-life scenario, an attacker creates a fake Word, Excel, or PowerPoint file containing hidden “mini‑programs” or special objects. They can run code and do other things on the affected computer. Normally, Office has safety checks that would block those mini-programs because they’re risky.
However, the vulnerability allows the attacker to tweak the file’s structure and hidden information in a way that tricks Office into thinking the dangerous mini‑program inside the document is harmless. As a result, Office skips the usual security checks and allows the hidden code to run.
As code to test the bypass is publicly available, increasing the risk of exploitation, users are under urgent advice to apply the patch.
What you need to do depends on which version of Office you’re using.
The affected products include Microsoft Office 2016, 2019, LTSC 2021, LTSC 2024, and Microsoft 365 Apps (both 32‑bit and 64‑bit).
Office 2021 and later are protected via a server‑side change once Office is restarted. To apply it, close all Office apps and restart them.
Office 2016 and 2019 require a manual update. Run Windows Update with the option to update other Microsoft products turned on.
If you’re running build 16.0.10417.20095 or higher, no action is required. You can check your build number by opening any Office app, going to your account page, and selecting About for whichever application you have open. Make sure the build number at the top reads 16.0.10417.20095 or higher.
What always helps:
We don’t just report on threats—we remove them
Cybersecurity risks should never spread beyond a headline. Keep threats off your devices by downloading Malwarebytes today.
The vulnerability is tracked as CVE-2026-21509 and it can be exploited to bypass security features.
The post Microsoft Patches Office Zero-Day Likely Exploited in Targeted Attacks appeared first on SecurityWeek.
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