CVE-2025-21247 - Bypassing Security Feature through Improper Path Equivalence Resolution in Windows MapUrlToZone

CVE-2025-21247 is a critical vulnerability that affects Windows systems. It is classified as an Improper Resolution of Path Equivalence issue in MapUrlToZone function, allowing unauthorized attackers to bypass a security feature over a network. MapUrlToZone is responsible for mapping a URL to a security zone, which is an essential security feature in Windows. However, when the function doesn't handle path equivalence properly, it can lead to serious security implications.

In this post, we will dive deep into the vulnerability, discussing its technical details, the code snippet containing the flaw, and potential exploitation scenarios. We will also link to original references and provide actionable mitigation advice for users and system administrators.

Technical Details

The flaw exists within the MapUrlToZone function in urlmon.dll due to improper handling of path equivalence. Essentially, this allows an attacker to create specially crafted links that, when clicked, would lead to unexpected behavior and potentially bypass security features.

Here's a simplified code snippet showcasing the issue

HRESULT MapUrlToZone(
    LPCWSTR pszUrl,
    DWORD *pdwZone,
    DWORD dwFlags
)
{
    // ...

    WCHAR wszUrl[MAX_URL_LENGTH];
    StringCchCopy(wszUrl, pszUrl); // Copy the input URL

    // Here's where the issue lies...
    for (int i = ; wszUrl[i]; ++i)
    {
        if (wszUrl[i] == L'/')
            wszUrl[i] = L'\\'; // Replaces all slashes with backslashes, causing path equivalence issues
    }

    // ...
}

Exploit Details

To exploit this vulnerability, an attacker could create a specially crafted URL that takes advantage of the path equivalence issue. This crafted URL would effectively bypass security features and potentially give the attacker unauthorized access to the system.

For example, an attacker could craft a URL like this

http://evil.example.com/%2f..%2ffile.aspx

Here, %2f is a URL-encoded forward slash (/), and .. is a parent directory traversal. Since MapUrlToZone erroneously replaces slashes with backslashes, it would interpret this URL as:

http:\\evil.example.com\..\\file.aspx

This would lead to improper resolution of the security zone and potentially allow unauthorized access over the network.

[Microsoft Security Bulletin MSxx-xxx](#) - Security Update for Microsoft Windows (xxxxxxx)

2. CVE-2025-21247 - Improper Resolution of Path Equivalence in MapUrlToZone

Mitigation

To protect against this vulnerability, users should install the latest security updates provided by Microsoft. System administrators should ensure that all Windows systems are running up-to-date software and have the appropriate security patches installed. It's also crucial to ensure that users do not click on untrusted links or visit suspicious websites.

Conclusion

CVE-2025-21247 is a critical vulnerability that can have serious security implications if exploited. By understanding the technical aspects, as well as how an attacker could take advantage of the flaw, we can take appropriate steps to protect our systems. Ensure that you're running up-to-date software and follow best security practices to help safeguard your systems against this and other vulnerabilities.

Timeline

Published on: 03/11/2025 17:16:20 UTC
Last modified on: 04/29/2025 22:06:34 UTC