CVE-2026-55040 - Microsoft SharePoint Weak Authentication Vulnerability

Technical Severity

CVSS v3.1 Metrics

CRITICAL

9.1 / 10

Minimal Risk Critical

Vector Specification

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Exploitation Likelihood

EPSS Prediction

39.65 %

Predictive Probability

Percentile Rank

98.5 th

Documented as more likely to be exploited than 98.5% of known CVEs.

Detection Date

Aug 18, 2026

Remediation Due

Aug 21, 2026

CISA Catalog Active

Threat Analysis

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

Remediation Directive

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.

External Intelligence

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040

CISA

CISA Advisory

https://www.cisa.gov/news-events/directives/bod-26-04-prioritizing-security-updates-based-risk

CISA

CISA Advisory

https://www.cisa.gov/news-events/directives/bod-26-04-implementation-guidance-prioritizing-security-updates-based-risk

NVD

National Vulnerability Database

https://nvd.nist.gov/vuln/detail/CVE-2026-55040