CVE-2026-21511 Microsoft Outlook Spoofing Vulnerability
An attacker can exploit untrusted data deserialization in Microsoft Outlook to perform spoofing attacks over the network.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-21511?
CVE-2026-21511 is a high-severity unsafe deserialization vulnerability in Microsoft 365 Apps, published on 10 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Deserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.
Weakness: CWE-502
Which versions are affected by CVE-2026-21511?
It affects 5 products.
Products include Microsoft 365 Apps, Microsoft Office, Microsoft Office Long Term Servicing Channel, Microsoft Sharepoint Server, Microsoft Word.
How do I fix CVE-2026-21511?
No fix has been published for CVE-2026-21511 yet. Limit exposure of the affected software until one is available.
Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.
Is CVE-2026-21511 being exploited?
There is no known exploitation of CVE-2026-21511 in the wild and no public exploit code.
Other vulnerabilities in Microsoft 365 Apps
- CVE-2024-21413 Microsoft Outlook Improper Input Validation
- CVE-2023-23397 Microsoft Office Outlook Privilege Escalation
- CVE-2024-38189 Microsoft Project Remote Code Execution Vulnerability
- CVE-2023-35311 Microsoft Outlook Security Feature Bypass
- CVE-2026-21514 Microsoft Office Word Reliance on Untrusted Inputs in a Security Decision
Sources
Also known as GHSA-x27j-2rq3-h4mw.