CVE-2025-49740: Windows 10 1507 Auth Bypass Vulnerability

CVE-2025-49740 Overview

CVE-2025-49740 is a security feature bypass vulnerability affecting Windows SmartScreen, a critical protection mechanism designed to warn users about potentially malicious files and applications downloaded from the internet. The vulnerability allows an unauthorized attacker to bypass SmartScreen protections over a network, potentially enabling malicious content to execute without the expected security warnings.

This vulnerability is classified as CWE-693 (Protection Mechanism Failure), indicating a fundamental flaw in the security control that SmartScreen provides. The attack can be initiated remotely over a network and requires user interaction, such as clicking a malicious link or opening a crafted file.

Critical Impact

Successful exploitation allows attackers to bypass Windows SmartScreen protections, enabling malicious files to execute without warning users, significantly increasing the risk of malware delivery and system compromise.

Affected Products

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

Discovery Timeline

• July 8, 2025 - CVE-2025-49740 published to NVD
• July 17, 2025 - Last updated in NVD database

Technical Details for CVE-2025-49740

Vulnerability Analysis

The vulnerability exists in the Windows SmartScreen protection mechanism, which is responsible for evaluating downloaded files and warning users about potentially dangerous content. SmartScreen typically inspects files marked with the Mark of the Web (MOTW) attribute and checks them against Microsoft's reputation database.

This protection mechanism failure allows an attacker to craft content that circumvents SmartScreen's evaluation process entirely. When successfully exploited, malicious files can be executed without triggering the standard SmartScreen warning dialogs that would normally alert users to potential threats.

The vulnerability requires user interaction for exploitation, meaning an attacker must convince a target to open a malicious file or navigate to a malicious URL. However, the bypass nature of this vulnerability makes social engineering attacks significantly more dangerous, as the built-in security warnings that users rely on are suppressed.

Root Cause

The root cause is classified as CWE-693: Protection Mechanism Failure. This indicates that the SmartScreen security feature fails to properly protect against malicious content under certain conditions. The mechanism that should evaluate and warn about potentially dangerous files contains a flaw that allows the protection to be completely bypassed when specific conditions are met during network-based content delivery.

Attack Vector

The attack vector is network-based, requiring user interaction for successful exploitation. An attacker could deliver malicious content through various channels:

The vulnerability can be exploited when a victim accesses attacker-controlled content over a network. This could involve clicking a link in a phishing email, downloading a file from a malicious website, or opening a document that references remote malicious resources. Once the SmartScreen bypass is triggered, the attacker's payload executes without the expected security warnings that would normally alert the user to the threat.

The attack does not require authentication or elevated privileges to initiate, making it accessible to a wide range of threat actors. The impact affects confidentiality, integrity, and availability of the compromised system.

Detection Methods for CVE-2025-49740

Indicators of Compromise

• Execution of downloaded files without corresponding SmartScreen warning events in Windows Defender logs
• Unusual process execution chains originating from browser downloads or email attachments without MOTW-related delays
• Windows Security event log entries indicating SmartScreen evaluation failures or unexpected bypasses

Detection Strategies

• Monitor for file executions that bypass expected SmartScreen verification, particularly files downloaded from external sources
• Implement endpoint detection rules that flag executable or script files running immediately after download without SmartScreen events
• Analyze network traffic for patterns associated with SmartScreen bypass techniques, including unusual file download behaviors

Monitoring Recommendations

• Enable enhanced logging for Windows Defender SmartScreen events (Event IDs 1000-1002 in Microsoft-Windows-SmartScreen/Debug)
• Deploy SentinelOne agents with Behavioral AI to detect post-bypass malicious activity patterns
• Monitor for execution of files from common download locations (Downloads folder, browser cache directories) without preceding SmartScreen evaluation events

How to Mitigate CVE-2025-49740

Immediate Actions Required

• Apply the latest Microsoft security updates from the July 2025 Patch Tuesday release immediately
• Enable Windows Defender Application Guard where possible for additional isolation of browser-downloaded content
• Implement application allowlisting to reduce the impact of SmartScreen bypass attacks
• Educate users about increased vigilance for suspicious downloads until patches are deployed

Patch Information

Microsoft has released security updates addressing this vulnerability. Organizations should consult the Microsoft Security Update Guide for CVE-2025-49740 for detailed patch information and download links specific to their affected Windows versions.

Patches are available for all affected Windows 10 and Windows 11 versions, as well as Windows Server 2016 through Windows Server 2025. Enterprise environments should prioritize deployment through Windows Server Update Services (WSUS) or Microsoft Endpoint Configuration Manager.

Workarounds

• Configure Windows Defender Application Control (WDAC) policies to restrict execution of unsigned or untrusted applications
• Implement strict email gateway filtering to block potentially malicious attachments before they reach end users
• Use Microsoft Defender for Endpoint's Attack Surface Reduction (ASR) rules to block executable content from email clients and webmail
• Consider temporarily blocking high-risk file types at the network perimeter until patches are deployed

# Enable Attack Surface Reduction rules via PowerShell to mitigate risk
Set-MpPreference -AttackSurfaceReductionRules_Ids BE9BA2D9-53EA-4CDC-84E5-9B1EEEE46550 -AttackSurfaceReductionRules_Actions Enabled
# Block executable content from email client and webmail
Set-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled