CVE-2026-55040

Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.

Score

CVSS 3.1 9.1 Critical

Status

Analyzed - secure@microsoft.com

Published

2026-07-14

Last modified

2026-10-28

Known exploited (CISA KEV)

since 2026-08-18, action due 2026-08-21: 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.

Weaknesses

CWE-1390

Affected products

• Sharepoint Server (Microsoft)
• Sharepoint Server 2016 (Microsoft)
• Sharepoint Server 2019 (Microsoft)

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/