CVE-2018-5174: Mozilla Firefox

High severity, CVSS 7.5. EPSS: 1.8% chance of exploitation in the next 30 days.

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.

Affected products

• Mozilla Firefox: before 52.8.0 (fixed in 52.8.0); before 60.0 (fixed in 60.0)
• Mozilla Thunderbird: before 52.8.0 (fixed in 52.8.0)
• Mozilla Thunderbird ESR: before 52.8.0 (fixed in 52.8.0)

Published 2018-06-11. Last modified 2026-06-17.