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
- 69.54% probability · 99th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Listed 18 August 2026 · due 21 August 2026
- Weakness
- CWE-1390
- Affected
- microsoft/sharepoint server
- Source
- secure@microsoft.com
EPSS moved from 17.5% on 24 September 2026 to 69.5% on 30 October 2026. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”.
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, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.