CVE-2021-27076 in SharePoint Server
Summary
by MITRE • 03/11/2021
Microsoft SharePoint Server Remote Code Execution Vulnerability
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Analysis
by VulDB Data Team • 08/21/2026
Microsoft SharePoint Server contains a remote code execution vulnerability that arises from improper input validation in the server-side rendering functionality. This flaw exists in the way SharePoint processes certain user-supplied data within its web parts and controls, specifically when handling serialized objects and complex data structures. The vulnerability stems from insufficient sanitization of input parameters that are passed through the server-side rendering pipeline, allowing attackers to inject malicious code that executes with the privileges of the SharePoint application pool account. This represents a critical security gap that aligns with CWE-20, which identifies improper input validation as a fundamental weakness in software systems. The vulnerability is particularly concerning because it enables attackers to bypass standard security controls and execute arbitrary code on the affected server.
The technical exploitation of CVE-2021-27076 occurs when an attacker crafts malicious input that gets processed through SharePoint's server-side rendering engine. This typically involves manipulating web part properties or data feeds that are then rendered on the server. The flaw allows for deserialization attacks where crafted payloads can trigger remote code execution without requiring authentication or elevated privileges. Attackers can leverage this vulnerability to deploy malware, establish persistence mechanisms, or escalate privileges within the SharePoint environment. The attack surface extends across multiple SharePoint components including the web application layer, the server-side rendering engine, and potentially the underlying database connections. This vulnerability is classified under the MITRE ATT&CK framework as part of the T1566 technique for credential access through exploitation of remote services, and T1059 for command and script injection. The exploitability is enhanced by the fact that SharePoint Server is commonly deployed in enterprise environments where network segmentation may be insufficient to prevent lateral movement.
The operational impact of this vulnerability extends far beyond simple code execution, as it provides attackers with extensive access to enterprise data and infrastructure. Successful exploitation can result in complete compromise of the SharePoint server, enabling attackers to access sensitive documents, user credentials, and other confidential information stored within the system. The vulnerability also poses risks to the broader network infrastructure since SharePoint servers often serve as central repositories for enterprise data and may contain privileged access to other systems. Organizations may experience significant business disruption, data loss, and regulatory compliance violations if this vulnerability is exploited successfully. The attack can lead to persistent backdoors, data exfiltration, and potential compromise of other connected systems through the SharePoint server acting as a gateway. This vulnerability particularly affects organizations with large SharePoint deployments where the attack surface is extensive and the potential for cascading security failures is high.
Organizations should implement immediate mitigations including applying the relevant security patches from Microsoft, which address the input validation issues in SharePoint Server's rendering engine. Network segmentation should be enhanced to limit access to SharePoint servers, particularly restricting direct internet access to these systems. Implementing proper input validation at multiple layers, including application-level controls and web application firewalls, can help detect and prevent exploitation attempts. Security monitoring should be enhanced to detect anomalous behavior in SharePoint server activities, including unusual data access patterns and code execution attempts. Regular security assessments and penetration testing should be conducted to identify potential exploitation vectors and validate the effectiveness of implemented controls. Additionally, organizations should consider implementing privileged access management solutions to reduce the potential impact of successful exploitation, ensuring that SharePoint server accounts operate with minimal necessary privileges. The vulnerability demonstrates the critical importance of maintaining up-to-date security patches and implementing defense-in-depth strategies to protect enterprise collaboration platforms from sophisticated attacks.