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CVE-2026-55040: Microsoft SharePoint Server Security Feature Bypass Vulnerability

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: yes — in CISA KEV since 2026-08-18
• EPSS: 69.5% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-07-14 · updated 2026-10-28
• Weakness: CWE-1390

Description

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

Summary

CVE-2026-55040 is a critical weak authentication flaw in Microsoft SharePoint Server that lets an unauthenticated attacker bypass a security feature over the network (CVSS 9.1). It is listed in CISA's Known Exploited Vulnerabilities catalog with active exploitation, a public exploit or detection template, and a 69.5% EPSS probability of exploitation within 30 days. SharePoint Server deployments are affected; the issue is fixed in version 16.0.19725.20434. Apply the vendor update or mitigations per CISA BOD 26-04 guidance, assess internet exposure of each asset, and discontinue use if mitigations are unavailable.

تُعد CVE-2026-55040 ثغرة ضعف في المصادقة ضمن Microsoft SharePoint Server، وتتيح لمهاجم غير مصرح له تجاوز إحدى ميزات الأمان عبر الشبكة (درجة CVSS 9.1 حرجة). الثغرة مدرجة في كتالوج CISA للثغرات المعروفة بالاستغلال مع استغلال نشط، ويتوفر استغلال عام أو قالب كشف، مع احتمال استغلال 69.5% خلال 30 يوماً وفق EPSS. تتأثر عمليات نشر SharePoint Server، وتم إصلاح المشكلة في الإصدار 16.0.19725.20434. طبّق تحديث المورّد أو التخفيفات وفق إرشادات CISA BOD 26-04، وقيّم تعرض كل أصل للإنترنت، وتوقف عن استخدام المنتج إذا لم تتوفر التخفيفات.

Affected products

• Microsoft SharePoint Server: before 16.0.19725.20434; 2016, 2019 — fixed in 16.0.19725.20434
• Microsoft SharePoint Server Subscription Edition: from 16.0.0 before 16.0.19725.20434 — fixed in 16.0.19725.20434
• Microsoft Sharepoint Enterprise Server 2016: from 16.0.0 before 16.0.5561.1001 — fixed in 16.0.5561.1001
• Microsoft SharePoint Server: from 16.0.0 before 16.0.10417.20175 — fixed in 16.0.10417.20175

How to fix

Upgrade to a fixed version: Microsoft SharePoint Server 16.0.19725.20434; Microsoft SharePoint Server Subscription Edition 16.0.19725.20434; Microsoft Sharepoint Enterprise Server 2016 16.0.5561.1001; Microsoft SharePoint Server 16.0.10417.20175. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55040
• https://github.com/sfewer-r7/CVE-2026-55040
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-55040
• https://www.rapid7.com/blog/post/ra-microsoft-sharepoint-jwt-token-authentication-bypass-cve-2026-55040/

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