CVE-2023-21742: Microsoft SharePoint Foundation RCE Flaw

CVE-2023-21742 Overview

CVE-2023-21742 is a Remote Code Execution (RCE) vulnerability affecting Microsoft SharePoint Server. This vulnerability allows an authenticated attacker with low privileges to execute arbitrary code on affected SharePoint installations through a network-based attack. Due to the improper access control mechanisms (CWE-284), attackers can potentially gain complete control over affected SharePoint servers, leading to full compromise of confidentiality, integrity, and availability.

Critical Impact

Authenticated attackers can achieve remote code execution on vulnerable SharePoint servers, potentially compromising enterprise collaboration environments and sensitive corporate data.

Affected Products

• Microsoft SharePoint Foundation 2013 SP1
• Microsoft SharePoint Server 2013 SP1 (Enterprise)
• Microsoft SharePoint Server 2016 (Enterprise)
• Microsoft SharePoint Server 2019
• Microsoft SharePoint Server Subscription Edition

Discovery Timeline

• January 10, 2023 - CVE-2023-21742 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-21742

Vulnerability Analysis

This Remote Code Execution vulnerability in Microsoft SharePoint Server stems from improper access control (CWE-284), allowing authenticated users to bypass security restrictions and execute arbitrary code on the server. The vulnerability requires network access and valid authentication credentials, but only low-level privileges are needed to exploit it. Once exploited, an attacker can achieve complete compromise of the SharePoint server without any user interaction.

The attack complexity is low, meaning no specialized conditions or complex techniques are required beyond basic authentication. The potential impact spans all three security domains—confidentiality, integrity, and availability—all rated as high impact. This indicates that successful exploitation could result in complete disclosure of sensitive information stored in SharePoint, modification of critical business documents and configurations, and disruption of SharePoint services.

Root Cause

The vulnerability originates from improper access control mechanisms within Microsoft SharePoint Server. Specifically, the application fails to properly validate user permissions before allowing certain operations that can lead to code execution. This access control weakness enables authenticated users with minimal privileges to perform actions that should be restricted to highly privileged administrators or the system itself.

Attack Vector

The attack is network-based, requiring the attacker to have authenticated access to the vulnerable SharePoint Server. The exploitation path involves:

1. An attacker establishes authenticated network connectivity to the target SharePoint Server
2. The attacker leverages their low-privilege credentials to access vulnerable functionality
3. Through the improper access control flaw, the attacker bypasses authorization checks
4. The attacker submits specially crafted requests that trigger code execution on the server
5. Arbitrary code executes in the context of the SharePoint application, potentially leading to full server compromise

The vulnerability does not require user interaction, making it particularly dangerous in environments where multiple users have authenticated access to SharePoint. For detailed technical information and exploitation specifics, refer to the Microsoft Security Update CVE-2023-21742.

Detection Methods for CVE-2023-21742

Indicators of Compromise

• Unusual process spawning from SharePoint worker processes (w3wp.exe) executing unexpected commands or scripts
• Suspicious file creation or modification in SharePoint directories, particularly web shells or unauthorized executables
• Anomalous network traffic originating from SharePoint servers to external or internal destinations
• Unexpected authentication patterns with low-privilege accounts accessing sensitive SharePoint administrative functions

Detection Strategies

• Monitor SharePoint ULS (Unified Logging Service) logs for error patterns or access control violations indicating exploitation attempts
• Implement endpoint detection rules to identify suspicious process chains originating from IIS worker processes serving SharePoint
• Deploy network intrusion detection signatures to identify malformed or suspicious requests targeting SharePoint endpoints
• Utilize SentinelOne's behavioral AI to detect anomalous code execution patterns on SharePoint servers

Monitoring Recommendations

• Enable verbose logging on SharePoint servers and forward logs to a centralized SIEM for correlation and analysis
• Configure alerts for failed authentication followed by successful exploitation patterns
• Monitor for lateral movement indicators from SharePoint servers to other network resources
• Implement file integrity monitoring on critical SharePoint directories and configuration files

How to Mitigate CVE-2023-21742

Immediate Actions Required

• Apply the Microsoft security update for CVE-2023-21742 immediately to all affected SharePoint Server installations
• Review authentication logs to identify any potentially compromised accounts that may have been used for exploitation
• Conduct a security audit of SharePoint permissions to ensure principle of least privilege is enforced
• Implement network segmentation to limit the blast radius if a SharePoint server is compromised

Patch Information

Microsoft has released security updates to address CVE-2023-21742. Organizations should apply the appropriate patch for their SharePoint version as documented in the Microsoft Security Update Guide. The patch addresses the underlying access control weakness that enables remote code execution.

Affected versions requiring updates:

• SharePoint Foundation 2013 SP1
• SharePoint Server 2013 SP1 Enterprise
• SharePoint Server 2016 Enterprise
• SharePoint Server 2019
• SharePoint Server Subscription Edition

Workarounds

• Restrict network access to SharePoint servers using firewall rules to limit exposure to trusted networks only
• Review and reduce the number of authenticated users with access to SharePoint, removing unnecessary accounts
• Implement Web Application Firewall (WAF) rules to detect and block suspicious request patterns targeting SharePoint
• Enable enhanced auditing on SharePoint servers to improve visibility into potential exploitation attempts

# Example: Restrict SharePoint access using Windows Firewall (adjust IP ranges as needed)
netsh advfirewall firewall add rule name="Block External SharePoint Access" dir=in action=block protocol=tcp localport=443 remoteip=any
netsh advfirewall firewall add rule name="Allow Trusted Network SharePoint Access" dir=in action=allow protocol=tcp localport=443 remoteip=10.0.0.0/8,172.16.0.0/12,192.168.0.0/16