CVE-2021-28474: SharePoint Foundation RCE Vulnerability

CVE-2021-28474 Overview

CVE-2021-28474 is a Remote Code Execution vulnerability affecting Microsoft SharePoint Server. This vulnerability allows authenticated attackers with low privileges to execute arbitrary code on vulnerable SharePoint installations through network-based attacks without requiring user interaction. The flaw is categorized under CWE-436 (Interpretation Conflict), indicating an issue with how SharePoint interprets certain input that can be exploited for malicious purposes.

Critical Impact

Authenticated attackers can achieve remote code execution on vulnerable Microsoft SharePoint Server installations, potentially compromising the confidentiality, integrity, and availability of enterprise collaboration environments and sensitive data stored within SharePoint.

Affected Products

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

Discovery Timeline

• 2021-05-11 - CVE-2021-28474 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-28474

Vulnerability Analysis

This Remote Code Execution vulnerability in Microsoft SharePoint Server stems from an interpretation conflict (CWE-436) in how SharePoint processes certain requests. The vulnerability allows an authenticated user with basic permissions to craft malicious requests that exploit the interpretation conflict, ultimately leading to arbitrary code execution on the server.

According to the Zero Day Initiative Advisory ZDI-21-574, the flaw exists within the handling of server-side requests. The vulnerability requires only low-privilege authentication to exploit, making it particularly dangerous in environments where many users have basic SharePoint access. Once exploited, attackers can achieve complete compromise of the SharePoint server, gaining access to all stored documents, user credentials, and potentially pivoting to other systems in the network.

Root Cause

The root cause of CVE-2021-28474 lies in an interpretation conflict within SharePoint Server's request processing logic. When SharePoint receives specially crafted input, there is a discrepancy in how different components interpret the data, creating a security gap that attackers can leverage. This type of vulnerability occurs when one component parses input differently than another, allowing attackers to bypass security controls by exploiting the inconsistency in interpretation.

Attack Vector

The attack is network-based and requires the attacker to have low-privilege authenticated access to the SharePoint environment. The exploitation does not require any user interaction, meaning that once an attacker has valid credentials (even limited ones), they can directly exploit the vulnerability. The attack complexity is low, making it accessible to attackers with moderate technical capabilities.

An attacker would typically authenticate to SharePoint with minimal credentials, then craft and submit malicious requests that exploit the interpretation conflict. Upon successful exploitation, the attacker's code executes with the privileges of the SharePoint application pool, typically granting extensive system access.

The vulnerability mechanism involves crafting requests that exploit how SharePoint interprets certain server-side operations. For detailed technical analysis, refer to the Microsoft Security Advisory CVE-2021-28474 and Zero Day Initiative Advisory ZDI-21-574.

Detection Methods for CVE-2021-28474

Indicators of Compromise

• Unusual SharePoint application pool process activity, including spawning child processes or network connections to external hosts
• Anomalous requests to SharePoint server-side endpoints from authenticated users
• Unexpected file system modifications within SharePoint directories or web application folders
• Suspicious PowerShell or command-line execution originating from SharePoint worker processes (w3wp.exe)

Detection Strategies

• Monitor SharePoint ULS logs and Windows Event logs for unusual error patterns or access attempts to sensitive server-side functionality
• Implement network traffic analysis to detect anomalous outbound connections from SharePoint servers
• Deploy endpoint detection and response (EDR) solutions such as SentinelOne to detect suspicious process creation chains from SharePoint application pools
• Review authentication logs for patterns indicating credential abuse or privilege escalation attempts

Monitoring Recommendations

• Enable and centralize SharePoint audit logging for all user activities, especially those involving server-side operations
• Configure alerts for unusual process behavior on SharePoint servers, particularly processes spawned by w3wp.exe
• Implement regular vulnerability scanning to identify unpatched SharePoint installations
• Monitor for reconnaissance activities targeting SharePoint version endpoints

How to Mitigate CVE-2021-28474

Immediate Actions Required

• Apply the Microsoft security update for CVE-2021-28474 immediately on all affected SharePoint installations
• Conduct an inventory of all SharePoint Server deployments to identify vulnerable versions (2013 SP1, 2016, 2019)
• Review SharePoint access permissions and implement least-privilege principles to limit potential attacker access
• Isolate vulnerable SharePoint servers if immediate patching is not possible

Patch Information

Microsoft has released security updates to address this vulnerability as part of their May 2021 security release. Administrators should download and apply the appropriate patches from the Microsoft Security Advisory CVE-2021-28474. The advisory contains version-specific patches for SharePoint Foundation 2013 SP1, SharePoint Server 2016 Enterprise, and SharePoint Server 2019.

Workarounds

• Restrict SharePoint access to only trusted users and networks while awaiting patch deployment
• Implement network segmentation to isolate SharePoint servers from critical infrastructure
• Enable enhanced monitoring and logging on SharePoint servers to detect potential exploitation attempts
• Consider temporarily disabling non-essential SharePoint features that may expose additional attack surface

# Verify SharePoint patch level using PowerShell
Get-SPFarm | Select BuildVersion
# Check installed SharePoint updates
Get-SPProduct -Local