Microsoft SharePoint Weak Authentication Vulnerability

[ Why it matters ]Priority signals

• Exploited in the wild: on CISA's Known Exploited Vulnerabilities list since 2026-08-18
• EPSS: 70% chance of exploitation in the next 30 days (higher than 99% of CVEs)
• Critical severity: CVSS 9.1

[ Description ]Official description

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

CISA: Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.

[ Fixes ]Fixes and patches

CISA required action (US federal deadline 21 Aug 2026): 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.

• Patch on msrc.microsoft.comVendor patch or bulletin · NVD · Reference tagged 'Patch' by NVD

[ References ]References

• msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040PatchVendor Advisory
• github.com/sfewer-r7/CVE-2026-55040Exploit
• www.cisa.gov/known-exploited-vulnerabilities-catalogExploitThird Party Advisory
• www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-20…US Government Resource