Microsoft SmartScreen

Microsoft SmartScreen is a phishing and malware filter implemented in several Microsoft products including Internet Explorer, Microsoft Edge and Outlook.com, as well as Windows 8 and later. It is designed to help protect users against attacks that utilize social engineering and drive-by downloads to infect a system by scanning URLs accessed by a user against a blacklist of websites containing known threats. A detailed FAQ by Microsoft on the SmartScreen filter is available at this link.

SmartScreen was first introduced in Internet Explorer 7 then known as the Phishing Filter. Phishing Filter does not check every website visited by the user, only those that are known to be suspicious.

With the release of Internet Explorer 8, the Phishing Filter was renamed to SmartScreen and extended to include protection from socially engineered malware. Every website and download is checked against a local list of popular legitimate websites; if the site is not listed, the entire address is sent to Microsoft for further checks. If it has been labeled as an impostor or harmful, Internet Explorer 8 will show a screen prompting that the site is reported harmful and shouldn't be visited. From there the user can either visit their homepage, visit the previous site, or continue to the unsafe page. If a user attempts to download a file from a location reported harmful, then the download is cancelled. The effectiveness of SmartScreen filtering has been reported to be superior to socially engineered malware protection in other browsers.

According to Microsoft, the SmartScreen technology used by Internet Explorer 8 was successful against phishing or other malicious sites and in blocking of socially engineered malware.

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Wikipedia