CVE-2023-21725: Windows MSRT Privilege Escalation Flaw

CVE-2023-21725 Overview

CVE-2023-21725 is an elevation of privilege vulnerability affecting the Windows Malicious Software Removal Tool (MSRT). This vulnerability allows a locally authenticated attacker to exploit race conditions and symbolic link handling flaws to gain elevated privileges on the target system. The vulnerability requires local access and involves complex timing conditions, but successful exploitation could result in significant integrity and availability impacts.

Critical Impact

Local attackers with low privileges can exploit race conditions and symlink vulnerabilities to elevate their privileges, potentially compromising system integrity and availability.

Affected Products

• Microsoft Windows Malicious Software Removal Tool (all versions prior to patch)

Discovery Timeline

• January 10, 2023 - CVE-2023-21725 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-21725

Vulnerability Analysis

This vulnerability stems from improper handling of symbolic links (symlinks) combined with a race condition (Time-of-Check Time-of-Use or TOCTOU) within the Windows Malicious Software Removal Tool. The vulnerability is classified under CWE-59 (Improper Link Resolution Before File Access) and CWE-362 (Concurrent Execution Using Shared Resource with Improper Synchronization).

The Windows Malicious Software Removal Tool performs file operations during its scanning and remediation processes. When the tool processes files or directories, there exists a window between when it checks file permissions or attributes and when it actually performs operations on those files. An attacker can exploit this timing gap by manipulating symbolic links to redirect file operations to privileged locations.

The local attack vector requires the attacker to have already established a foothold on the target system with low-privilege access. The high attack complexity stems from the precise timing required to win the race condition and successfully place a symbolic link during the vulnerable window.

Root Cause

The root cause of CVE-2023-21725 lies in two interconnected weaknesses:

1. Improper Symbolic Link Resolution (CWE-59): The MSRT does not properly validate or restrict symbolic links before performing file operations. This allows an attacker to create symlinks that point to sensitive system files or directories.

2. Race Condition (CWE-362): A TOCTOU race condition exists where the tool checks file properties at one point in time but performs operations at a later time. During this window, an attacker can swap legitimate files or directories with malicious symbolic links.

When these two weaknesses are combined, a low-privileged user can manipulate the tool's file operations to affect files they would not normally have access to modify or delete.

Attack Vector

The attack requires local access to the system and involves the following general approach:

An attacker with a low-privileged account monitors for MSRT execution or triggers it manually. During the tool's operation, the attacker creates symbolic links in locations where MSRT performs file operations. By carefully timing the creation of these symlinks to occur between the security check and the actual file operation, the attacker can redirect write, delete, or modify operations to privileged system locations. This can result in denial of service through file deletion or corruption, or potentially privilege escalation through modification of security-sensitive files.

The attack complexity is high because the attacker must precisely time their actions to exploit the narrow TOCTOU window. However, automated tools and repeated attempts can increase the likelihood of success.

Detection Methods for CVE-2023-21725

Indicators of Compromise

• Unusual symbolic link creation in temporary directories or directories accessed by MSRT
• Unexpected file operations or deletions in system directories coinciding with MSRT execution
• Process monitoring showing rapid symlink creation and deletion patterns
• Event logs indicating privilege escalation attempts following MSRT execution

Detection Strategies

• Monitor for symbolic link creation in directories commonly used by MSRT such as %TEMP% and Windows Defender directories
• Implement file integrity monitoring on critical system files to detect unauthorized modifications
• Enable advanced audit policies for object access and privilege use
• Use endpoint detection solutions to identify suspicious TOCTOU attack patterns

Monitoring Recommendations

• Enable Windows Security Event logging for file system operations (Event IDs 4656, 4663, 4660)
• Configure SentinelOne's real-time file monitoring to detect symlink manipulation attempts
• Monitor scheduled task execution for MSRT and correlate with file system activity
• Implement behavioral analysis to detect rapid file attribute changes characteristic of race condition exploits

How to Mitigate CVE-2023-21725

Immediate Actions Required

• Update the Windows Malicious Software Removal Tool to the latest patched version from Microsoft
• Review systems for signs of exploitation using the detection strategies outlined above
• Ensure endpoint protection solutions are updated with the latest threat intelligence
• Restrict unnecessary local user accounts and enforce principle of least privilege

Patch Information

Microsoft has released a security update addressing CVE-2023-21725. Organizations should apply the latest version of the Windows Malicious Software Removal Tool. The tool typically updates automatically through Windows Update, but administrators should verify that updates are being applied correctly across their environment.

For detailed patch information and download links, refer to the Microsoft Security Advisory CVE-2023-21725.

Workarounds

• Temporarily disable automatic execution of MSRT via Group Policy until patches can be applied
• Restrict user permissions to create symbolic links using Local Security Policy (SeCreateSymbolicLinkPrivilege)
• Implement application control policies to limit which users can execute MSRT
• Use SentinelOne's Singularity platform to monitor and block suspicious symlink manipulation attempts

# Restrict symbolic link creation privileges via PowerShell
# Run as Administrator
# This limits which users can create symbolic links
secedit /export /cfg c:\temp\secpol.cfg
# Review and modify SeCreateSymbolicLinkPrivilege in the exported policy
# Then reimport with: secedit /configure /db c:\windows\security\local.sdb /cfg c:\temp\secpol.cfg