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CVE-2026-55040: Microsoft SharePoint Weak Authentication Vulnerability

CVE-2026-55040 is rated critical with a CVSS v3.1 base score of 9.1 and affects Microsoft SharePoint. Per its scoring, it can be exploited over the network, without authentication, without any user interaction. CISA added it to its Known Exploited Vulnerabilities catalog on 2026-08-18, which means exploitation in the wild has been observed.

Vulnerability description

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

Severity breakdown

CVSS v3.1 9.1: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack vector: Network. Exploitable remotely over the network, with no need for local or physical access.
Attack complexity: Low. No special conditions; the attack can be repeated reliably.
Privileges required: None. No account or prior access is needed.
User interaction: None. No action from any user is needed.
Scope: Unchanged. Impact stays within the vulnerable component.
Confidentiality impact: High. Total loss of confidentiality; sensitive data can be fully disclosed.
Integrity impact: High. Total loss of integrity; data or system state can be fully modified.
Availability impact: None. No effect on availability.

Underlying weakness

CWE-1390: Weak Authentication

The product uses an authentication mechanism to restrict access to specific users or identities, but the mechanism does not sufficiently prove that the claimed identity is correct.

Affected products

Microsoft Sharepoint Server: before 16.0.19725.20434; 2016; 2019