CVE-2024-21318: Microsoft SharePoint Server RCE Flaw

CVE-2024-21318 Overview

CVE-2024-21318 is a remote code execution vulnerability affecting Microsoft SharePoint Server. This vulnerability allows an authenticated attacker with Site Owner privileges to execute arbitrary code on the SharePoint server through insecure deserialization. The flaw stems from improper handling of serialized data, enabling attackers to inject malicious payloads that execute in the context of the SharePoint application pool identity.

Critical Impact

Successful exploitation allows authenticated attackers to execute arbitrary code on affected SharePoint servers, potentially leading to complete server compromise, data exfiltration, and lateral movement within enterprise networks.

Affected Products

• Microsoft SharePoint Server Subscription Edition
• Microsoft SharePoint Server 2019
• Microsoft SharePoint Server 2016 Enterprise

Discovery Timeline

• 2024-01-09 - CVE-2024-21318 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-21318

Vulnerability Analysis

This vulnerability is classified under CWE-502 (Deserialization of Untrusted Data), a well-known vulnerability class that has historically affected enterprise applications. The insecure deserialization flaw in SharePoint Server allows authenticated users with elevated privileges (Site Owner level) to craft malicious serialized objects that, when processed by the server, result in arbitrary code execution.

The attack requires network access and authenticated credentials with Site Owner permissions, but does not require user interaction. Successful exploitation can lead to complete compromise of confidentiality, integrity, and availability of the affected system, allowing attackers to access sensitive corporate data, modify SharePoint content, and potentially pivot to other systems within the network.

Root Cause

The root cause of CVE-2024-21318 lies in SharePoint Server's failure to properly validate and sanitize serialized data before processing. When the application deserializes user-controlled input without adequate type checking or allowlisting, attackers can supply specially crafted serialized objects containing malicious payloads. These payloads are then instantiated and executed during the deserialization process, bypassing normal security controls.

Attack Vector

The attack vector is network-based, requiring an authenticated attacker with Site Owner privileges to submit malicious serialized data to a vulnerable SharePoint endpoint. The attacker crafts a serialized object containing a payload that, when deserialized by the SharePoint server, triggers code execution. Since Site Owner privileges are required, the attack surface is limited to users who have been granted elevated permissions within the SharePoint environment.

The exploitation does not require any user interaction beyond the attacker's own actions. Once the malicious payload is submitted and deserialized, arbitrary code executes in the security context of the SharePoint application pool, which typically has significant access to the server and connected resources.

Detection Methods for CVE-2024-21318

Indicators of Compromise

• Unusual process spawning from SharePoint application pool processes (w3wp.exe)
• Unexpected network connections originating from SharePoint server components
• Suspicious serialized data patterns in SharePoint web requests and logs
• Anomalous file system modifications or new file creation in SharePoint directories

Detection Strategies

• Monitor Windows Event Logs for process creation events where parent process is associated with SharePoint IIS application pools
• Implement web application firewall (WAF) rules to detect and block suspicious serialization patterns in HTTP requests
• Enable and review SharePoint Unified Logging System (ULS) logs for deserialization-related errors or anomalies
• Deploy endpoint detection and response (EDR) solutions to monitor SharePoint server behavior for post-exploitation activities

Monitoring Recommendations

• Configure SIEM alerts for unusual process chains originating from SharePoint services
• Monitor outbound network traffic from SharePoint servers for connections to unknown or suspicious external endpoints
• Audit Site Owner permission assignments regularly to ensure principle of least privilege
• Enable enhanced logging for SharePoint web applications to capture detailed request information

How to Mitigate CVE-2024-21318

Immediate Actions Required

• Apply the Microsoft security update for CVE-2024-21318 immediately on all affected SharePoint Server installations
• Review and audit Site Owner permissions across all SharePoint sites to minimize the potential attacker pool
• Implement network segmentation to limit SharePoint server access to trusted networks only
• Enable enhanced monitoring on SharePoint servers until patches can be applied

Patch Information

Microsoft has released security updates to address CVE-2024-21318. Administrators should obtain the appropriate patches from the Microsoft Security Update Guide. The patches address the underlying deserialization vulnerability by implementing proper validation of serialized data before processing.

Organizations should prioritize patching based on the exposure of their SharePoint environments and follow Microsoft's guidance for deploying updates to SharePoint Server infrastructure.

Workarounds

• Restrict Site Owner permissions to only essential personnel until patches can be applied
• Implement additional network access controls to limit connectivity to SharePoint servers from untrusted networks
• Consider enabling additional authentication requirements for Site Owner-level operations
• Deploy web application firewall rules to filter potentially malicious serialization payloads

# Review SharePoint Site Owner permissions using SharePoint Management Shell
Get-SPSite | ForEach-Object { 
    Write-Host "Site: $($_.Url)"
    $_.RootWeb.SiteAdministrators | ForEach-Object { 
        Write-Host "  Site Owner: $($_.LoginName)" 
    }
}