CVE-2018-5174 in Firefox
Summary
by MITRE
In the Windows 10 April 2018 Update, Windows Defender SmartScreen honors the "SEE_MASK_FLAG_NO_UI" flag associated with downloaded files and will not show any UI. Files that are unknown and potentially dangerous will be allowed to run because SmartScreen will not prompt the user for a decision, and if the user is offline all files will be allowed to be opened because Windows won't prompt the user to ask what to do. Firefox incorrectly sets this flag when downloading files, leading to less secure behavior from SmartScreen. Note: this issue only affects Windows 10 users running the April 2018 update or later. It does not affect other Windows users or other operating systems. This vulnerability affects Thunderbird < 52.8, Thunderbird ESR < 52.8, Firefox < 60, and Firefox ESR < 52.8.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2018-5174 represents a critical security flaw in the Windows 10 April 2018 Update that fundamentally undermines the security posture of the operating system's SmartScreen feature. This issue stems from the improper handling of the SEE_MASK_FLAG_NO_UI flag during file download operations, creating a significant bypass in the system's security controls. The vulnerability specifically affects Windows 10 users who have installed the April 2018 update or later versions, rendering the SmartScreen protection mechanism ineffective for certain file execution scenarios. The core problem manifests when Windows Defender SmartScreen encounters downloaded files that have the SEE_MASK_FLAG_NO_UI flag set, causing the security feature to suppress all user interface prompts and notifications that would normally alert users to potentially dangerous files.
The technical flaw in this vulnerability can be categorized under CWE-693 Protection Mechanism Failure, where a security control fails to properly execute its intended protective function. When Firefox incorrectly sets the SEE_MASK_FLAG_NO_UI flag during file downloads, it inadvertently disables the user interface components that SmartScreen relies upon to make security decisions. This flag essentially instructs the operating system to suppress all user interface elements associated with the file operation, which includes the crucial security prompts that would normally warn users about potentially malicious files. The vulnerability operates at the intersection of browser security and operating system security controls, creating a scenario where legitimate security mechanisms are bypassed through improper flag handling by third-party applications.
The operational impact of this vulnerability extends beyond simple security bypass to create a dangerous environment where unknown and potentially malicious files can execute without user consent or awareness. When users are offline, the vulnerability becomes even more concerning as all files are automatically permitted to run without any security verification, effectively removing all protective barriers that SmartScreen was designed to provide. This behavior creates a persistent risk for users who may unknowingly execute malicious payloads that would have otherwise been blocked by SmartScreen's security prompts. The vulnerability affects not only Windows 10 users but also impacts the broader ecosystem of applications that rely on SmartScreen for protection, including Thunderbird and Firefox browsers that are configured to set this flag during downloads.
The attack surface for this vulnerability aligns with ATT&CK technique T1059.001 Command and Scripting Interpreter, as attackers can leverage this bypass to execute malicious code through legitimate browser download mechanisms without triggering the expected security warnings. This vulnerability also relates to T1070.004 Indicator Removal on Host, as the absence of security prompts makes it easier for malware to operate undetected. The affected software versions including Firefox < 60 and Thunderbird < 52.8 demonstrate how this issue propagates through the software ecosystem, where browser implementations that incorrectly handle system flags can compromise the security of the entire operating system. Organizations and individuals running these vulnerable versions face increased risk of malware infection, data theft, and other security incidents that could have been prevented by proper SmartScreen operation.
Mitigation strategies for CVE-2018-5174 require both immediate and long-term approaches to address the root cause of the vulnerability. The primary recommendation involves updating all affected software to versions that properly handle the SEE_MASK_FLAG_NO_UI flag, including Firefox 60, Thunderbird 52.8, and the latest Windows 10 updates. System administrators should implement network-level controls to monitor and restrict file downloads from untrusted sources, while also ensuring that all systems are kept up to date with the latest security patches. Organizations should consider implementing additional security controls such as application whitelisting and endpoint detection and response solutions to provide layered protection against exploitation of this vulnerability. The fix requires careful attention to how browser applications interact with system security features, emphasizing the importance of proper flag handling and the need for comprehensive testing of security-related code changes in third-party applications.