Microsoft · SharePoint
CVE-2026-55040
Microsoft SharePoint Server Security Feature Bypass Vulnerability
What it is
Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
CISA: Microsoft SharePoint contains a weak authentication vulnerability which allows an unauthorized attacker to bypass a security feature over a network.
What to do
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.
Deadline for US federal agencies: August 21, 2026
More exploited flaws from Microsoft
| Added | Vulnerability | Product | Severity | Attack chance |
|---|---|---|---|---|
| Sep 25, 2026 | Code Injection | SharePoint | High 8.8 | 8.2% |
| Jul 22, 2026 | Deserialization of Untrusted Data Vulnerability | SharePoint | Critical 9.8 | 3.0% |
| Jul 16, 2026 | Deserialization of Untrusted Data | SharePoint | Critical 9.8 | 16% |
| Mar 18, 2026 | Deserialization of Untrusted Data | SharePoint | Critical 9.8 | 29% |
| Jul 22, 2025 | Code Injection | SharePoint | High 8.8 | >99.9% |
| Jul 22, 2025 | Improper Authentication | SharePoint | Medium 6.5 | 99.1% |
| Jul 20, 2025 | Deserialization of Untrusted Data | SharePoint | Critical 9.8 | >99.9% |
| Oct 22, 2024 | Deserialization | SharePoint | High 7.2 | 51% |
Sources: Known Exploited Vulnerabilities (CISA, public domain), CVE records (cve.org) and EPSS (FIRST). The chance of attacks is the EPSS estimate that someone will try to exploit the flaw in the next 30 days.