Published: June 2024
Severity: High
Component: Windows MapUrlToZone API
Overview
A newly disclosed Windows vulnerability, CVE-2025-21247, highlights a serious issue in how the Windows MapUrlToZone security feature resolves file paths. An attacker can craft paths that trick this security feature, enabling unauthorized actions across network shares. This improper path handling can allow attackers to bypass important security controls, putting systems at risk—especially in enterprise environments relying on MapUrlToZone for zone-based access controls.
What is MapUrlToZone?
MapUrlToZone is a Windows API function used by browsers and other applications to map a specified URL or file path to a "zone" (like Internet, Intranet, Trusted, or Local Machine zones). Windows then applies security policies based on that zone. This API is widely used for enforcing network and file path restrictions.
The Vulnerability
CVE-2025-21247 is an *improper path equivalence vulnerability* in MapUrlToZone. In short, Windows fails to recognize that certain alternative or encoded file paths are equivalent to legitimate restricted paths. The function can be tricked by:
Alternative path formats (\\?\UNC\sharename\folder\file.txt)
- Dot-dot slashes (../ or ..\)
Encoding tricks (like %2e%2e\)
By crafting paths with these tricks, a remote attacker can bypass zone restrictions, making Windows treat a network resource as if it’s trusted or local—even when it shouldn’t be.
Host a malicious file on a network share or web server.
2. Craft a misleading path linking to the malicious file, using dot segments or path encoding to hide the real location.
3. Trick a user or application into processing the malicious file (through social engineering or some automated process).
4. MapUrlToZone is called, but because of the path trickery, it returns a more trusted zone than it should.
5. Security features are bypassed — for instance, a file from the Internet is treated like it’s from the Intranet or even the local machine.
Example Code Snippet
Below is a demonstration in Python (using ctypes) of how a Windows program might incorrectly classify a path using this vulnerability:
import ctypes
from ctypes import wintypes
url = r'\\?\UNC\SHARE\hidden\..\malicious.exe' # This should resolve to \SHARE\malicious.exe
# MapUrlToZone importing:
wininet = ctypes.WinDLL('wininet')
MapUrlToZone = wininet.MapUrlToZoneW
MapUrlToZone.argtypes = [wintypes.LPCWSTR, ctypes.POINTER(ctypes.c_ulong), wintypes.DWORD]
MapUrlToZone.restype = wintypes.DWORD
zone = ctypes.c_ulong()
result = MapUrlToZone(url, ctypes.byref(zone), )
# Print zone mapping (should be Internet, but may report Intranet/Local due to bug)
print(f'Zone for path is: {zone.value}')
If Windows is vulnerable, this misleading path may incorrectly return zone 1 (Intranet) or (Local), rather than the correct external zone.
Monitor for unexplained zone mappings in application logs or audit logs.
Microsoft Patch:
Microsoft has released an official advisory with a patch for this issue.
References
- Microsoft CVE-2025-21247 Advisory
- Windows MapUrlToZone API Documentation
- OWASP Path Traversal
Final Notes
CVE-2025-21247 is a perfect reminder that path normalization is critical when enforcing security controls. If you’re using MapUrlToZone, double-check your inputs and update your systems immediately. For defenders, watch your logs and educate your teams about this new bypass technique.
Stay safe and keep your Windows environments patched!
Timeline
Published on: 03/11/2025 17:16:20 UTC
Last modified on: 04/29/2025 22:06:34 UTC