CVE-2026-33110 - Microsoft SharePoint Deserialization Vulnerability

CVE-2026-33110 is a critical vulnerability affecting Microsoft Office SharePoint. The vulnerability stems from the deserialization of untrusted data, which can be exploited by an authorized attacker to achieve remote code execution (RCE) on the affected system. Successful exploitation of this vulnerability allows an attacker to execute arbitrary code within the context of the SharePoint application, potentially leading to complete system compromise. The vulnerability was published on 2026-05-12 and requires an attacker to be authenticated, but requires no user interaction. This RCE vulnerability can allow attackers to pivot to other systems and gain access to sensitive data within the organization.

Attack Chain

1. An authorized attacker gains initial access to the SharePoint environment via valid credentials.
2. The attacker crafts a malicious payload containing serialized data designed to exploit the deserialization vulnerability (CVE-2026-33110).
3. The attacker injects the malicious payload into a SharePoint component that processes serialized data.
4. When SharePoint processes the crafted input, it attempts to deserialize the data.
5. The deserialization process triggers the execution of arbitrary code embedded within the malicious payload.
6. The attacker’s code executes with the privileges of the SharePoint application pool.
7. The attacker leverages the code execution to install a webshell for persistent access or perform lateral movement.
8. The attacker uses the compromised SharePoint server as a pivot point to access other systems on the network and exfiltrate sensitive information.

Impact

Successful exploitation of CVE-2026-33110 can lead to complete compromise of the SharePoint server, potentially impacting all sites and data hosted on the platform. An attacker could gain access to sensitive documents, modify content, or disrupt services for all users. Due to the central role SharePoint often plays in document management and collaboration, this vulnerability represents a significant risk to data confidentiality, integrity, and availability within an organization. The vulnerability could also be used as a stepping stone to compromise other systems within the network, leading to a widespread security breach.

Recommendation

• Apply the security update released by Microsoft to address CVE-2026-33110 on all affected SharePoint servers immediately. Refer to the Microsoft Security Response Center (MSRC) advisory for specific patching instructions: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-33110.
• Deploy the Sigma rule “Detect SharePoint Suspicious Deserialization Attempt” to identify potential exploitation attempts based on unusual process execution.
• Monitor SharePoint logs for suspicious activity related to deserialization processes as an additional layer of defense.