CVE-2026-55040

Published Jul 14, 2026

Last updated 18 days ago

Exploit known CVSS critical 9.1

Microsoft Office SharePoint

Jwt

Zero-day

Overview

AI description

Automated description summarized from trusted sources.

CVE-2026-55040 is an authentication bypass vulnerability affecting Microsoft SharePoint Server. This flaw stems from issues within the JSON Web Token (JWT) validation pipeline, allowing a remote, unauthenticated attacker to bypass the authentication process. By exploiting this vulnerability, an attacker can assume the identity of any SharePoint site user, provided they know the target user's Active Directory Security ID (SID) or User Principal Name (UPN). This authentication bypass can be chained with other vulnerabilities to achieve further compromise, such as unauthenticated remote code execution. The vulnerability was discovered by Rapid7 Labs during a zero-day research project.

Description: Weak authentication in Microsoft Office SharePoint allows an unauthorized attacker to bypass a security feature over a network.
Source: secure@microsoft.com
NVD status: Analyzed
Products: sharepoint_server

Risk scores

CVSS 3.1

Type: Secondary
Base score: 9.1
Impact score: 5.2
Exploitability score: 3.9
Vector string: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Severity: CRITICAL

Known exploits

Data from CISA

Vulnerability name: Microsoft SharePoint Weak Authentication Vulnerability
Exploit added on: Aug 18, 2026
Exploit action due: Aug 21, 2026
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.

Weaknesses

secure@microsoft.com: CWE-1390

Hype score: Not currently trending

Configurations

[
  {
    "nodes": [
      {
        "cpeMatch": [
          {
            "criteria": "cpe:2.3:a:microsoft:sharepoint_server:*:*:*:*:subscription:*:*:*",
            "matchCriteriaId": "5FA63EA8-B225-4E35-A6A3-DBDECF5AC4C8",
            "versionEndExcluding": "16.0.19725.20434",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:a:microsoft:sharepoint_server:2016:*:*:*:enterprise:*:*:*",
            "matchCriteriaId": "F815EF1D-7B60-47BE-9AC2-2548F99F10E4",
            "vulnerable": true
          },
          {
            "criteria": "cpe:2.3:a:microsoft:sharepoint_server:2019:*:*:*:*:*:*:*",
            "matchCriteriaId": "6122D014-5BF1-4AF4-8B4D-80205ED7785E",
            "vulnerable": true
          }
        ],
        "negate": false,
        "operator": "OR"
      }
    ]
  }
]

References

Sources include official advisories and independent security research.