CVE-2026-55040
Published 2026-07-14 · Updated 2026-08-19 · Source secure@microsoft.com
Summary
CVE-2026-55040 is a critical-severity (CVSS 9.1) vulnerability in Microsoft SharePoint. CISA added it to the Known Exploited Vulnerabilities catalog on August 18, 2026, which means exploitation in the wild has been confirmed; US federal agencies had to remediate it by August 21, 2026. CISA does not currently list it as used in ransomware campaigns. Microsoft has 390 vulnerabilities in the KEV catalog, 117 of them linked to ransomware.
All 390 known-exploited Microsoft vulnerabilities →Description
Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
CISA Known-Exploited Vulnerability
Product: Microsoft — SharePoint
Name: Microsoft SharePoint Weak Authentication Vulnerability
Date added: 2026-08-18 · Due: 2026-08-21
Required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040
- https://github.com/sfewer-r7/CVE-2026-55040
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040
- https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/