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Critical Microsoft SharePoint Vulnerability Allows Remote Code Execution via SafeControls Bypass

A vulnerability tracked as CVE-2026-65660 with a CVSS score of 8.8 affects Microsoft SharePoint Server 2016, SharePoint Server 2019, and Subscription Edition. The flaw permits an authenticated low-privileged attacker to achieve arbitrary code execution on on-premises servers. It stems from improper handling of quotes in attacker-controlled Register directives within the ToolPane component, bypassing the SafeControls mechanism. This allows registration of dangerous .NET classes followed by deserialization-based remote code execution. Microsoft released patches on August 11, 2026, and initially reported no public disclosure or exploitation. Publication of a detailed technical analysis has now increased the risk for unpatched installations.

BoletimSecTranslated from Portuguese

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A vulnerability in Microsoft SharePoint Server allows an authenticated attacker with low privileges to execute arbitrary code on the server. Identified as CVE-2026-65660, the flaw received a CVSS score of 8.8.

The affected products are SharePoint Server 2016, SharePoint Server 2019, and the Subscription Edition, meaning on-premises deployments maintained within organizations' own infrastructure.

The issue lies in an injection that bypasses the SafeControls protection, a mechanism intended to restrict which classes can be registered and executed by the application. Exploitation occurs in the ToolPane component. When processing attacker-controlled Register directives, quotes in the content are not handled properly, allowing extra directives to be inserted after security validations have passed.

This enables the attacker to register dangerous .NET classes and achieve arbitrary code execution through deserialization, starting from a standard user account without requiring administrative privileges.

Microsoft published fixes on August 11, 2026, with specific versions for each product line. At the time of disclosure, the company stated the flaw had not been publicly disclosed or exploited in attacks. This situation changed with the release of a detailed technical analysis of the vulnerability.

With the mechanics of the flaw now documented, the risk of exploitation has increased for organizations still running unpatched servers.

Microsoft SharePointSharePoint Server 2019SharePoint Server 2016CVE-2026-65660ToolPane
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