Known exploited · Microsoft
CVE-2026-20963: Microsoft SharePoint Deserialization of Untrusted Data Vulnerability
What it is
Microsoft SharePoint contains a deserialization of untrusted data vulnerability that allows an unauthorized attacker to execute code over a network.
- Vendor
- Microsoft
- Product
- SharePoint
- Added to KEV
- March 18, 2026
- CISA due date
- March 21, 2026
- Ransomware use
- Unknown
What CISA requires
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Forensic triage
No
What to do now
- Find every instance of SharePoint you run, including the forgotten ones, and check its version against the vendor advisory.
- Apply the vendor's fix or mitigation, before March 21, 2026 if you are bound by CISA's deadline, sooner if it faces the internet.
- Assume it was tried against you already: look for signs of exploitation from the date it was added, not from the day you patched.
- Until every instance is patched, block the attack at the edge: a WAF in blocking mode stops Java deserialization payloads.
References
The Dome
Block it at the edge while you patch
The Dome's WAF runs the OWASP Core Rule Set in blocking mode and stops Java deserialization payloads before they reach your application.
See the DomeT Academy
Train the engineers who fix it
Security engineering on real hosts in the browser, every objective checked on the machine. The first two lessons of Linux 1 are free.
Start freeSource: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.