CVE-2021-31963: Microsoft SharePoint Foundation RCE Flaw

CVE-2021-31963 Overview

CVE-2021-31963 is a Remote Code Execution (RCE) vulnerability affecting Microsoft SharePoint Server. This vulnerability allows authenticated attackers with low privileges to execute arbitrary code on affected SharePoint installations via network-based attacks. The flaw exists within the SharePoint Server and SharePoint Foundation products, posing a significant risk to enterprise environments that rely on these collaboration platforms.

Critical Impact

Successful exploitation of this vulnerability enables remote code execution on affected Microsoft SharePoint servers, potentially allowing attackers to compromise the underlying server, access sensitive data, and pivot to other resources within the network.

Affected Products

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

Discovery Timeline

• June 8, 2021 - CVE-2021-31963 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-31963

Vulnerability Analysis

This Remote Code Execution vulnerability in Microsoft SharePoint Server allows authenticated attackers to execute arbitrary code within the context of the SharePoint application pool. The vulnerability requires network access and low-level authentication to exploit, meaning an attacker needs valid credentials to the SharePoint environment but does not require administrative privileges. No user interaction is required for successful exploitation, making this vulnerability particularly dangerous in enterprise environments where SharePoint serves as a critical collaboration platform.

The impact of successful exploitation is severe, affecting confidentiality, integrity, and availability of the targeted system. An attacker who successfully exploits this vulnerability could gain full control over the SharePoint server, potentially accessing sensitive corporate documents, modifying content, or disrupting business operations.

Root Cause

Microsoft has not disclosed detailed information about the specific root cause of this vulnerability (classified as NVD-CWE-noinfo). However, based on the nature of the vulnerability as a remote code execution flaw, it likely involves improper handling of user-supplied input within SharePoint server-side components, allowing attackers to inject and execute malicious code.

Attack Vector

The attack vector for CVE-2021-31963 is network-based, requiring an authenticated attacker with low privileges to send specially crafted requests to a vulnerable SharePoint server. The exploitation process typically involves:

1. Obtaining valid credentials to access the SharePoint environment (even limited guest or member-level access may suffice)
2. Crafting malicious requests that exploit the underlying vulnerability
3. Sending the payload to the target SharePoint server
4. Achieving code execution within the SharePoint application context

Since no proof-of-concept code has been publicly verified for this vulnerability, technical details about the specific exploitation mechanism remain limited. For detailed technical information, refer to the Microsoft Security Advisory CVE-2021-31963.

Detection Methods for CVE-2021-31963

Indicators of Compromise

• Unexpected processes spawning from w3wp.exe (IIS worker process) associated with SharePoint application pools
• Unusual outbound network connections originating from SharePoint servers
• Suspicious entries in SharePoint ULS logs indicating abnormal request patterns
• Unauthorized file modifications or new files appearing in SharePoint installation directories

Detection Strategies

• Monitor SharePoint IIS logs for anomalous HTTP requests targeting SharePoint endpoints
• Implement application-level logging to capture detailed request parameters and response behaviors
• Deploy endpoint detection and response (EDR) solutions like SentinelOne to detect post-exploitation activities
• Utilize network traffic analysis to identify command-and-control communications from SharePoint servers

Monitoring Recommendations

• Enable Windows Security Event logging on SharePoint servers, particularly for process creation (Event ID 4688)
• Configure SharePoint diagnostic logging at increased verbosity to capture detailed application behavior
• Implement SIEM correlation rules to detect patterns consistent with SharePoint exploitation attempts
• Regularly review authentication logs for suspicious login patterns to SharePoint environments

How to Mitigate CVE-2021-31963

Immediate Actions Required

• Apply Microsoft security updates for SharePoint Server immediately
• Review SharePoint authentication logs for signs of unauthorized access
• Implement network segmentation to limit exposure of SharePoint servers
• Consider temporarily restricting access to SharePoint from untrusted networks until patching is complete

Patch Information

Microsoft has released security updates to address this vulnerability as part of their June 2021 security releases. Affected organizations should apply the appropriate updates for their SharePoint version:

• SharePoint Foundation 2013 SP1 - Apply the June 2021 cumulative update
• SharePoint Server 2013 SP1 Enterprise - Apply the June 2021 cumulative update
• SharePoint Server 2016 - Apply the June 2021 cumulative update
• SharePoint Server 2019 - Apply the June 2021 cumulative update

For specific patch downloads and detailed installation instructions, refer to the Microsoft Security Advisory CVE-2021-31963.

Workarounds

• Restrict network access to SharePoint servers using firewall rules and network access control lists
• Implement additional authentication requirements such as multi-factor authentication (MFA) to reduce the risk of credential-based attacks
• Consider placing SharePoint servers behind a web application firewall (WAF) with rules to detect and block common exploitation patterns
• Limit user permissions within SharePoint to the minimum necessary level following the principle of least privilege

# Example: Restrict SharePoint IIS bindings to specific IP ranges using PowerShell
# Review and test before implementing in production

# Import IIS Administration module
Import-Module WebAdministration

# Get current SharePoint site bindings
Get-WebBinding -Name "SharePoint - 80"

# Add IP restriction to SharePoint application pool (example)
# Configure IIS IP Address and Domain Restrictions feature
Set-WebConfigurationProperty -Filter "/system.webServer/security/ipSecurity" `
    -Name "allowUnlisted" -Value "false" -PSPath "IIS:\Sites\SharePoint - 80"