CVE-2025-21276: Windows 10 1507 DoS Vulnerability

CVE-2025-21276 Overview

CVE-2025-21276 is a denial of service vulnerability in the Windows MapUrlToZone function. The flaw stems from an integer underflow condition [CWE-191] in URL zone mapping logic used across supported Windows client and server platforms. A remote, unauthenticated attacker can trigger the condition over the network with low complexity and no user interaction. Successful exploitation results in high availability impact on the target system. Microsoft addressed the issue in the January 14, 2025 security update cycle.

Critical Impact

Remote, unauthenticated attackers can cause a denial of service condition on Windows systems by triggering an integer underflow in MapUrlToZone, affecting confidentiality-neutral but availability-critical workloads.

Affected Products

• Microsoft Windows 10 (1507, 1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (22H2, 23H2, 24H2)
• Microsoft Windows Server 2008, 2012, 2016, 2019, 2022, 2022 23H2, and 2025

Discovery Timeline

• 2025-01-14 - Microsoft releases security patch for CVE-2025-21276
• 2025-01-14 - CVE-2025-21276 published to NVD
• 2025-01-27 - Last updated in NVD database

Technical Details for CVE-2025-21276

Vulnerability Analysis

The vulnerability resides in the MapUrlToZone function, a Windows API used to map a URL into a security zone such as Internet, Intranet, Trusted, or Restricted Sites. Browser components, MOTW (Mark of the Web) handling, and other Windows security features rely on this function to make trust decisions.

An attacker can deliver a crafted URL or document referencing such a URL to a target host. When the host processes the input through MapUrlToZone, the integer underflow condition is triggered. The result is a process or service crash that produces a denial of service condition without leaking data or modifying system state.

Root Cause

The root cause is an integer underflow [CWE-191] in arithmetic logic inside MapUrlToZone. Specially crafted URL input causes an unsigned value calculation to wrap below zero, producing an unexpectedly large value used in subsequent processing. That value drives faulty memory or control-flow behavior that terminates the calling process.

Attack Vector

Exploitation occurs over the network and requires no authentication or user interaction. An attacker delivers a malicious URL through any vector that causes a Windows component to invoke MapUrlToZone on the input, such as a hosted webpage, a file with embedded URLs, or a network protocol that resolves remote resources. No verified public proof-of-concept exploit was available at publication.

No verified exploit code is available. See the Microsoft Security Update CVE-2025-21276 advisory for vendor technical details.

Detection Methods for CVE-2025-21276

Indicators of Compromise

• Unexpected crashes or restarts of explorer.exe, browser processes, or other components that perform URL zone evaluation
• Windows Error Reporting (WER) entries referencing faults in urlmon.dll, iertutil.dll, or related zone-mapping libraries
• Inbound network traffic delivering malformed or unusually structured URLs to user-facing endpoints

Detection Strategies

• Monitor application crash telemetry for repeat faults in processes that call MapUrlToZone
• Correlate URL-handling process terminations with preceding network requests or document opens
• Inspect web and email gateways for malformed URL patterns targeting Windows clients

Monitoring Recommendations

• Enable Windows Event Log collection for Application Error (Event ID 1000) and WER (Event ID 1001) events
• Forward crash and process termination telemetry to a centralized SIEM for trend analysis
• Track patch deployment status for the January 2025 cumulative update across all Windows endpoints and servers

How to Mitigate CVE-2025-21276

Immediate Actions Required

• Apply the January 14, 2025 Microsoft security update to all affected Windows 10, Windows 11, and Windows Server systems
• Prioritize internet-facing systems and user endpoints that process untrusted URLs and documents
• Verify patch installation through Windows Update history or enterprise patch management reporting

Patch Information

Microsoft published the official fix in the January 2025 security update cycle. Refer to the Microsoft Security Update CVE-2025-21276 advisory for KB numbers specific to each Windows version. Apply the cumulative update appropriate for each operating system build.

Workarounds

• No vendor-provided workaround replaces patching; install the cumulative update as soon as feasible
• Restrict inbound web traffic and filter untrusted URLs at the email and web gateway layers to reduce exposure until patching is complete
• Apply network segmentation to limit the blast radius of service outages on unpatched servers

# Verify January 2025 update installation on Windows
wmic qfe list brief /format:table | findstr /i "2025"

# PowerShell alternative to list installed updates
Get-HotFix | Where-Object { $_.InstalledOn -ge (Get-Date '2025-01-14') }