CVE-2025-27737: Windows 10 1507 Auth Bypass Vulnerability

CVE-2025-27737 Overview

CVE-2025-27737 is a security feature bypass vulnerability in Windows Security Zone Mapping. The flaw stems from improper input validation [CWE-20] within the component responsible for assigning content to security zones. An attacker who successfully exploits this vulnerability can bypass zone-based security controls locally, provided a user interacts with crafted content. Microsoft published the advisory on April 8, 2025, and the issue affects a broad range of Windows client and server releases, from Windows 10 1507 through Windows 11 24H2 and from Windows Server 2008 through Windows Server 2025.

Critical Impact
Successful exploitation lets an attacker bypass Windows Security Zone Mapping enforcement, exposing high-impact compromise of confidentiality, integrity, and availability on the targeted host.

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

2025-04-08 - CVE-2025-27737 published to NVD with Microsoft's security advisory
2025-07-10 - Last updated in NVD database

Technical Details for CVE-2025-27737

Vulnerability Analysis

Windows Security Zone Mapping classifies resources into zones such as Internet, Intranet, Trusted Sites, and Restricted Sites. The classification drives enforcement decisions across Internet Explorer mode, Microsoft Edge legacy components, the Attachment Manager, and Mark-of-the-Web (MOTW) handling. CVE-2025-27737 results from improper input validation within this mapping logic. Crafted input that the component fails to validate causes content to receive a less-restrictive zone assignment than intended.

The scope is marked as changed, meaning the bypass extends impact beyond the immediately vulnerable component. Downstream protections that trust zone assignments, including SmartScreen prompts, Protected View, and Office macro warnings, can be undermined when zone metadata is incorrect. User interaction is required, typically through opening a file or navigating to attacker-supplied content.

Root Cause

The root cause is insufficient validation of input passed to the Security Zone Mapping component. The component accepts data that should be rejected or normalized, producing zone determinations that do not reflect the true origin of the content.

Attack Vector

The attack vector is local and requires user interaction. An attacker delivers a crafted file, shortcut, or URL handler that the target opens. When Windows evaluates the content for zone placement, the validation flaw produces an incorrect lower-trust-to-higher-trust mapping. Subsequent security checks downstream of zone enforcement are then bypassed.

No public proof-of-concept code has been linked in the NVD record, and no entry exists on the CISA Known Exploited Vulnerabilities catalog at the time of writing.

Detection Methods for CVE-2025-27737

Indicators of Compromise

Files arriving from external sources that lack expected Zone.Identifier Alternate Data Streams or contain manipulated MOTW values.
Office documents and scripts executing without the Protected View or SmartScreen prompts normally triggered for Internet-zone content.
Unexpected child processes spawned from Explorer, browsers, or mail clients after a user opens an attachment or link.

Detection Strategies

Monitor creation and modification of Zone.Identifier ADS streams on user profile directories and Downloads folders.
Correlate file write events from browsers and mail clients with subsequent process executions that skip standard MOTW prompts.
Hunt for registry edits under HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap that reassign hosts to lower-trust zones.

Monitoring Recommendations

Ingest endpoint process, file, and registry telemetry into a centralized analytics platform for cross-host correlation.
Track Windows patch compliance for the April 2025 cumulative update across all affected SKUs listed in the advisory.
Alert on user-initiated execution of downloaded executables, scripts, and installers that bypass expected SmartScreen warnings.

How to Mitigate CVE-2025-27737

Immediate Actions Required

Apply the April 2025 Microsoft security updates referenced in the Microsoft CVE-2025-27737 Update advisory to all affected Windows builds.
Prioritize patching on workstations and Remote Desktop Session Hosts where users routinely open external files and links.
Validate that legacy Windows Server 2008, 2012, and 2012 R2 systems receive the corresponding Extended Security Updates.

Patch Information

Microsoft addressed CVE-2025-27737 in the April 8, 2025 security update cycle. Refer to the Microsoft Security Response Center advisory for the per-product KB numbers and download links. Confirm installation through wmic qfe list or the Get-HotFix PowerShell cmdlet.

Workarounds

Restrict end-user ability to execute files from the Internet zone through AppLocker or Windows Defender Application Control policies.
Configure Group Policy to enforce Protected View for files originating from the Internet and Restricted Sites zones.
Block inbound delivery of high-risk file types at the email gateway and web proxy until patches are deployed.

bash

# Verify patch installation on Windows hosts
Get-HotFix | Where-Object { $_.InstalledOn -ge (Get-Date "2025-04-08") } | Format-Table HotFixID, InstalledOn, Description

# Enforce Protected View for Internet-zone Office files via registry
reg add "HKCU\Software\Policies\Microsoft\Office\16.0\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f