CVE-2021-34520 in SharePoint Server
Summary
by MITRE • 07/15/2021
Microsoft SharePoint Server Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-34467, CVE-2021-34468.
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Analysis
by VulDB Data Team • 07/17/2021
Microsoft SharePoint Server contains a critical remote code execution vulnerability that arises from improper input validation within the server-side rendering functionality. This flaw exists in the way SharePoint processes certain file upload operations and subsequently renders them within the web interface. The vulnerability specifically affects the server-side includes processing mechanism where untrusted input is not properly sanitized before being executed as part of the rendering pipeline. Attackers can exploit this weakness by uploading malicious files that contain specially crafted payloads designed to bypass standard security controls. The vulnerability is particularly dangerous because it allows remote attackers to execute arbitrary code on the target server with the privileges of the SharePoint service account. This creates a significant attack surface that can lead to full system compromise and lateral movement within network environments. The flaw is classified as a server-side request forgery vulnerability that enables attackers to manipulate the application's processing logic and inject malicious code into the server execution context.
The technical implementation of this vulnerability stems from a lack of proper validation and sanitization of file content within SharePoint's document library processing subsystem. When users upload files to SharePoint document libraries, the system performs server-side operations to generate previews and thumbnails for various file types. The vulnerability occurs in the handling of specific file formats where the system does not adequately validate the content before processing it through the rendering engine. This allows attackers to upload files that contain embedded malicious code which gets executed during the preview generation process. The vulnerability is particularly insidious because it can be exploited through legitimate user upload functionality, making it difficult to detect through traditional network monitoring. The flaw exists in the server-side template processing engine that handles various file types including office documents, images, and other media formats. Attackers can leverage this vulnerability to execute commands on the SharePoint server, potentially leading to complete system compromise.
The operational impact of this vulnerability extends far beyond simple code execution, as it provides attackers with persistent access to critical enterprise infrastructure. Once exploited, attackers can establish backdoors, escalate privileges, and move laterally throughout the network to access additional systems and data repositories. The vulnerability affects organizations that rely heavily on SharePoint for document management and collaboration, making it a prime target for sophisticated threat actors. The exploitation process typically involves uploading a malicious file that triggers the vulnerable code path during preview generation, allowing the attacker to execute arbitrary commands on the server. This creates a persistent threat that can remain undetected for extended periods, enabling attackers to maintain access and continue data exfiltration or system manipulation. Organizations using SharePoint Server are particularly vulnerable because the platform is widely deployed across enterprises, and the vulnerability affects multiple versions and configurations of the software.
Mitigation strategies for this vulnerability should focus on immediate patch application and network-level protections. Microsoft has released security updates that address this specific vulnerability, and organizations should prioritize applying these patches to all affected SharePoint Server instances. Additionally, network segmentation and access controls should be implemented to limit exposure of SharePoint servers to untrusted networks. The implementation of web application firewalls and content filtering solutions can help detect and block malicious file uploads before they reach the vulnerable processing engine. Organizations should also consider disabling unnecessary file upload capabilities and implementing strict file type validation policies. Security monitoring should be enhanced to detect anomalous file processing patterns and unusual command execution on SharePoint servers. The vulnerability aligns with several attack techniques documented in the attack chain including initial access through file uploads and privilege escalation through command execution. This vulnerability demonstrates the importance of proper input validation and the potential consequences of insufficient security controls in enterprise collaboration platforms. Organizations should also conduct comprehensive security assessments of their SharePoint environments to identify additional vulnerabilities and ensure proper configuration of security controls.