CVE-2026-55040
Microsoft SharePoint Weak Authentication Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 21 August 2026). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
Description
Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
- CVSS 3.1
- 9.1 CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- EPSS
- 50.59% probability · 99th percentile
- CISA KEV
- Listed 18 August 2026 · due 21 August 2026
- Weakness
- CWE-1390
- Affected
- microsoft/sharepoint server
- Source
- secure@microsoft.com
CISA notes
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. https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040 ; BOD 26-04: https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk ; Forensics Triage Requirements: https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk ; https://nvd.nist.gov/vuln/detail/CVE-2026-55040
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040Patch, Vendor Advisory
- https://github.com/sfewer-r7/CVE-2026-55040Exploit
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040Exploit, Third Party Advisory
- https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/US Government Resource
Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.