WDAC

Security Advisory

Windows Update Stack Privilege Escalation Vulnerability (CVE-2025-21204) – PoC Released  

The flaw, disclosed by researchers at Cyberdom Blog, poses a significant risk to millions of Windows users and organizations relying on windows.

OEM Windows 
Severity HIGH 
CVSS Score 7.8 
CVEs CVE-2025-21204 
POC Available Yes 
Actively Exploited No 
Exploited in Wild No 
Advisory Version 1.0 

Overview 

A high-severity vulnerability in the Windows Update Stack, CVE-2025-21204, enables local attackers to escalate privileges to SYSTEM level by exploiting trusted path abuse through symbolic links. The flaw affects various versions of Windows 10, Windows 11, and Windows Server.

A working proof-of-concept (PoC) exploit has been publicly released by security researcher Elli Shlomo, increasing the urgency to patch. The issue is addressed in the April 2025 cumulative update KB5055523. 

Vulnerability Name CVE ID Product Affected Severity CVSS Score 
​Windows Update Stack Privilege Escalation  CVE-2025-21204 Windows  HIGH  7.8 

Technical Summary 

The vulnerability lies in how Windows Update processes such as MoUsoCoreWorker.exe and UsoClient.exe, which run with SYSTEM privileges, handle directory junctions. Attackers can delete the legitimate Tasks directory under C:\ProgramData\Microsoft\UpdateStack and replace it with a symbolic link pointing to an attacker-controlled path. This allows the execution of arbitrary code as SYSTEM without triggering traditional security mechanisms.

A public PoC developed by Elli Shlomo demonstrates this exploit using only native Windows features—no external binaries or code injection required. 

This opens the door for a range of attacks, including installing persistent malware, disabling security tools, or accessing sensitive data.

CVE ID System Affected Vulnerability Details Exploit Prerequisites Impact 
  CVE-2025-21204  Windows 10 (10.0.10240.0 < 10.0.10240.20978, etc.), Windows 11, Server Misuse of NTFS junctions allows local attackers to redirect C:\ProgramData\Microsoft\UpdateStack\Tasks to attacker-controlled locations. SYSTEM-level update processes follow these junctions and execute unauthorized code. Attackers must have local access and limited user privileges; no user interaction required   Local privilege escalation, Code execution 

Source: Cyberdom 

Recommendations: 

  • Apply the April 2025 cumulative update (KB5055523) immediately. 
  • Restrict ACLs on C:\ProgramData\Microsoft\UpdateStack. 
  • Use AppLocker or WDAC to block symbolic link creation in sensitive directories. 
  • Monitor file operations involving UpdateStack and inetpub, regardless of IIS presence. 
  • Detect attempts to create NTFS junctions targeting update directories. 

Conclusion: 
CVE-2025-21204 is an example of a rather low-level and impactful threat doing trusted path abuse rather than complex memory corruption. This vulnerability demonstrates how attackers will exploit trust assumptions built into the operating system via native components.

The only defenses available are to immediately patch and harden directory access controls to stop this low-level and minimally visible localized privilege escalation. 

References: 


 

Security Advisory

Zero-Day Vulnerability in Microsoft Sysinternals Tools  

Summary 

A critical 0-Day vulnerability has been identified in nearly all Microsoft Sysinternals tools, allowing attackers to exploit DLL injection techniques to execute arbitrary code. This presents a significant risk to IT administrators and developers who rely on these utilities for system analysis and troubleshooting.

OEM Microsoft 
Severity High 
Date of Announcement 2025-02-05 
CVEs Not Yet Assigned 
Exploited in Wild No 
Patch/Remediation Available No 
Advisory Version 1.0 
Vulnerability Name Zero-Day  

Overview 

Despite being reported to Microsoft over 90 days ago, the vulnerability remains unpatched, as Microsoft considers it a “defense-in-depth” issue rather than a critical security flaw. 

Vulnerability Name CVE ID Product Affected Severity Impact 
            zero-day  Not Yet Assigned Microsoft Sysinternals Tools (Process Explorer, Autoruns, Bginfo, and potentially others)          High Arbitrary Code Execution, Privilege Escalation, Malware Deployment 

Technical Summary 

The vulnerability is caused by improper handling of DLL loading paths in affected Sysinternals utilities. When these tools search for required DLLs, they follow a specific search order, which may include untrusted locations such as network shares or user-writable directories. 

The issue arises from how Sysinternals tools prioritize DLL search paths, favoring untrusted directories such as: 

  • The Current Working Directory (CWD) 
  • Network locations (e.g., shared drives) 
  • User-writable paths over secure system directories 

This flaw allows attackers to place a malicious DLL in the same directory as a Sysinternals executable, tricking the application into loading the rogue DLL instead of the legitimate system DLL. 

Exploit Workflow 

  1. Attacker crafts a malicious DLL (e.g., cryptbase.dll or TextShaping.dll) containing a payload such as a reverse shell, ransomware, or trojan. 
  1. The DLL is placed in the same directory as a vulnerable Sysinternals tool. 
  1. The user unknowingly executes the tool (e.g., Bginfo.exe or procexp.exe) from that directory. 
  1. The malicious DLL is loaded instead of the legitimate system DLL. 
  1. Attackers gains code execution with the privileges of the running process (potentially SYSTEM privileges if run with admin rights). 

Recommendations 

  1. Avoid Running Sysinternals Tools from Network Locations 
  • Always copy tools to a local trusted directory before execution. 
  • Disable execution of .exe files from network drives if feasible. 
  1. Restrict DLL Search Paths 
  • Use SafeDLLSearchMode to prioritize secure directories. 
  • Implement DLL redirection to force tools to load DLLs from trusted paths. 
  1. Implement Application Control Policies 
  • Use AppLocker or Windows Defender Application Control (WDAC) to block unauthorized DLLs from loading. 
  • Restrict execution of Sysinternals tools to trusted admin-only directories. 
  1. Verify DLL Integrity Before Execution 
  • Use SigCheck (Sysinternals) to ensure all loaded DLLs are digitally signed. 
  • Block execution of unsigned or suspicious DLLs in sensitive directories. 
  1. Monitor for Suspicious DLL Loading Behavior 
  • Enable Sysmon logging to detect anomalous DLL loads (Event ID 7). 
  • Monitor for executions of Sysinternals tools from network shares (Event ID 4688). 

Conclusion 

Despite being responsibly disclosed to Microsoft in October 2024, the vulnerability in Sysinternals tools remains unpatched as of February 2025. Microsoft classifies it as a “defense-in-depth” issue, dismissing it as non-critical, while security researchers highlight its severe impact on enterprises, especially those running tools from network shares. This leaves users reliant on manual mitigations to avoid exploitation.

The Sysinternals tools, developed by Microsoft, are a widely-utilized suite of utilities designed to provide in-depth insights into the processes, services, and configurations of Windows systems. 

References: 

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