CVE-2026-21509 Microsoft Office: login or permission bypass
In plain words: A door that should be locked is not: part of the system can be reached without a valid login or permission. The attacker gets in as a user or administrator without knowing a password.
Confirmed exploited by CISA on · US federal patch deadline Feb 16, 2026
What to do
- Install Microsoft's security update for Office. If no update exists, follow the vendor's workaround or stop using the product until one ships.
- Turn on automatic updates for the operating system and for Office, PDF and archive apps.
- Be careful with unexpected attachments — documents, .zip/.rar files and shortcuts are common bait.
- Use a normal (non-admin) account day to day and keep built-in protection like Microsoft Defender on.
Who is affected
Could reach you. This one is aimed at devices or apps of the kind people own.
What's at risk: Your files, saved passwords and accounts. Many of these let a program that is already running gain full administrator control.
The details
- Official name
- Microsoft Office Security Feature Bypass Vulnerability
- Product
- Microsoft Office
- CISA description
- Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.
- Type of flaw
- Login or permission bypass (CWE-807)
- Confirmed exploited
- Jan 26, 2026
- US federal patch deadline
- Feb 16, 2026
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Answering about CVE-2026-21509 ·
Questions about CVE-2026-21509
Is CVE-2026-21509 being exploited?
Yes. CISA added CVE-2026-21509 to its Known Exploited Vulnerabilities catalog on Jan 26, 2026, which means there is reliable evidence of real-world attacks.
How do I fix CVE-2026-21509?
Install Microsoft's security update for Office. If no update exists, follow the vendor's workaround or stop using the product until one ships. Turn on automatic updates for the operating system and for Office, PDF and archive apps.
Am I affected by CVE-2026-21509?
Only if you use Microsoft Office. This one is aimed at devices or apps of the kind people own.