CVE-2020-17118: Microsoft SharePoint Foundation RCE Flaw

CVE-2020-17118 Overview

CVE-2020-17118 is a Remote Code Execution (RCE) vulnerability affecting Microsoft SharePoint products. This vulnerability allows unauthenticated remote attackers to execute arbitrary code on affected SharePoint servers over the network without any user interaction required. Due to the network-accessible nature of SharePoint deployments and the potential for complete system compromise, this vulnerability poses a significant risk to enterprise environments.

Critical Impact

Unauthenticated remote attackers can execute arbitrary code on vulnerable SharePoint servers, potentially leading to complete system compromise, data theft, and lateral movement within enterprise networks.

Affected Products

• Microsoft SharePoint Foundation 2010 SP2
• Microsoft SharePoint Foundation 2013 SP1
• Microsoft SharePoint Server 2016 Enterprise
• Microsoft SharePoint Server 2019

Discovery Timeline

• December 10, 2020 - CVE-2020-17118 published to NVD
• August 28, 2025 - Last updated in NVD database

Technical Details for CVE-2020-17118

Vulnerability Analysis

This vulnerability exists within Microsoft SharePoint's request handling mechanisms. The flaw allows unauthenticated attackers to send specially crafted requests to a vulnerable SharePoint server, which are then processed in a way that enables arbitrary code execution within the context of the SharePoint application pool identity. Successful exploitation grants the attacker the same privileges as the SharePoint service account, which typically has elevated permissions within the hosting environment.

The vulnerability is particularly dangerous because it requires no authentication, making it accessible to any attacker who can reach the SharePoint server over the network. Enterprise SharePoint deployments often contain sensitive corporate data, making them high-value targets for threat actors seeking to exfiltrate intellectual property or establish persistence within a target network.

Root Cause

The root cause of CVE-2020-17118 stems from improper validation and handling of user-supplied input within Microsoft SharePoint. When processing certain requests, SharePoint fails to properly sanitize or validate input data before using it in a security-sensitive context. This allows attackers to inject and execute malicious code on the server. Microsoft has not disclosed specific technical details about the vulnerable component to prevent exploitation, classifying the weakness as "NVD-CWE-noinfo."

Attack Vector

The attack is conducted remotely over the network. An attacker can exploit this vulnerability by sending specially crafted HTTP requests to a vulnerable SharePoint server. The attack requires no prior authentication and no user interaction, making it highly exploitable in exposed SharePoint environments.

The exploitation flow typically involves:

1. The attacker identifies a vulnerable SharePoint server accessible over the network
2. A specially crafted malicious request is sent to the target server
3. SharePoint processes the request without proper validation
4. Arbitrary code execution occurs in the context of the SharePoint application pool
5. The attacker gains control over the SharePoint server with the privileges of the service account

Detection Methods for CVE-2020-17118

Indicators of Compromise

• Unexpected processes spawning from SharePoint application pool processes (w3wp.exe)
• Unusual outbound network connections from SharePoint servers to unknown external IP addresses
• Web server logs showing malformed or suspicious HTTP requests targeting SharePoint endpoints
• Creation of unexpected files or modifications to SharePoint installation directories

Detection Strategies

• Monitor SharePoint IIS logs for anomalous requests, particularly those with unusual payloads or targeting known vulnerable endpoints
• Deploy network-based intrusion detection rules to identify exploitation attempts against SharePoint servers
• Implement endpoint detection and response (EDR) solutions to detect suspicious process behavior on SharePoint hosts
• Correlate Windows Security event logs with SharePoint application logs to identify potential compromise indicators

Monitoring Recommendations

• Enable verbose logging on SharePoint servers and forward logs to a centralized SIEM for analysis
• Implement file integrity monitoring on SharePoint server directories to detect unauthorized changes
• Monitor for new scheduled tasks, services, or startup items on SharePoint servers that could indicate persistence mechanisms
• Review SharePoint service account activity for anomalous authentication events or privilege usage

How to Mitigate CVE-2020-17118

Immediate Actions Required

• Apply the Microsoft security update immediately to all affected SharePoint servers
• Restrict network access to SharePoint servers using firewalls and network segmentation until patching is complete
• Review SharePoint server logs for any signs of exploitation or compromise prior to patching
• Consider temporarily taking vulnerable SharePoint servers offline if immediate patching is not possible

Patch Information

Microsoft has released security updates to address CVE-2020-17118 as part of their December 2020 security updates. Organizations should obtain and apply the appropriate patches for their SharePoint version from the Microsoft Security Update Guide. The Microsoft Security Advisory provides version-specific guidance for SharePoint Foundation 2010 SP2, SharePoint Foundation 2013 SP1, SharePoint Server 2016, and SharePoint Server 2019.

Workarounds

• Implement network-level access controls to limit SharePoint server exposure to trusted networks only
• Enable Web Application Firewall (WAF) rules to filter potentially malicious requests to SharePoint endpoints
• Consider implementing reverse proxy configurations with additional security controls in front of SharePoint servers
• Monitor and limit the privileges of the SharePoint application pool identity to reduce the impact of potential compromise

# Example: Restrict SharePoint access at the network level
# Windows Firewall rule to limit SharePoint access to internal network only
netsh advfirewall firewall add rule name="Restrict SharePoint External Access" ^
    dir=in action=block protocol=tcp localport=80,443 ^
    remoteip=any localip=any ^
    profile=any enable=yes
# Note: Adjust the rule to allow specific trusted IP ranges as needed