CVE-2026-55040: Microsoft SharePoint Weak Authentication Vulnerability

In short

Microsoft SharePoint has a weakness classified as weak authentication (CWE-1390). CISA added it to its Known Exploited Vulnerabilities catalog on 2026-08-18. NVD lists CVSS 3.1 at 9.1 CRITICAL.

What it is

“Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.” — CISA KEV, retrieved 2026-11-01

“Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.” — NVD, retrieved 2026-11-01

Who is affected

Microsoft SharePoint. NVD lists the following versions as affected:

• Versions before 16.0.19725.20434
• Versions version 2016
• Versions version 2019

Exploitation status

CISA KEV added: 2026-08-18. Known ransomware campaign use: Not reported by CISA. Due date CISA set for US federal civilian agencies: 2026-08-21.

Scores

NVD CVSS 3.1: 9.1 CRITICAL; vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N; source [email protected].

EPSS by FIRST: 69.54%; percentile 99.36; score date 2026-10-31. EPSS estimates the probability of exploitation in the next 30 days.

CWE: CWE-1390.

Timeline

CISA 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.” — CISA KEV

What to do

• Confirm whether you run Microsoft SharePoint and which version.
• Apply the vendor update or mitigation in the advisory.
• Follow CISA’s required action.
• Review the vendor guidance on signs of compromise.
• Track the fix through completion.

References

• Patch, Vendor Advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040
• US Government Resource: https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/
• NVD detail
• CVE.org record
• CISA KEV catalog

Sources and dates

CISA Known Exploited Vulnerabilities Catalog, version 2026.10.30; retrieved 2026-11-01. CISA KEV data: CC0 1.0. No endorsement by CISA.

NVD retrieved: 2026-11-01. This product uses the NVD API but is not endorsed or certified by the NVD.

EPSS scores by FIRST; score date 2026-10-31.