SmartScreen (officially called Windows SmartScreen, Windows Defender SmartScreen and SmartScreen Filter in different places) is a cloud-based anti-phishing and anti-malware component included in several Microsoft products, including Windows 8 and later, Internet Explorer, Microsoft Edge and Outlook.com. 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. With the Windows 10 Creators Update, Microsoft placed the SmartScreen settings into the Windows Defender Security Center.
SmartScreen in Internet Explorer
Internet Explorer 7: Phishing Filter
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.
Internet Explorer 8: SmartScreen Filter
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.
Beginning with Internet Explorer 8, SmartScreen can be enforced using Group Policy.
Internet Explorer 9: Application Reputation
Building on top of the SmartScreen Filter introduced in Internet Explorer 8, Internet Explorer 9's protection against malware downloads is extended with SmartScreen Application Reputation that detects untrustworthy executables. This warns a person if they are downloading an executable program without a safe reputation, from a site that does not have a safe reputation.
Internet Explorer Mobile 10
SmartScreen filtering at the desktop level, performing reputation checks by default on any file or application downloaded from the Internet, was introduced in Windows 8. Similar to the way SmartScreen works in Internet Explorer 9, if the program does not have an established good reputation, the user is alerted that running the program may harm their computer.
When SmartScreen is left at its default settings, the administrator needs to launch and run the program.
Microsoft faced concerns surrounding the privacy, legality and effectiveness of the new system, suggesting that the automatic analysis of files (which involves sending a cryptographic hash of the file and the user's IP address to a server) could be used to build a database of users' downloads online, and that the use of the outdated SSL 2.0 protocol for communication could allow an attacker to eavesdrop on the data. In response, Microsoft later issued a statement noting that IP addresses were only being collected as part of the normal operation of the service and would be periodically deleted, that SmartScreen on Windows 8 would only use SSL 3.0 for security reasons, and that information gathered via SmartScreen would not be used for advertising purposes or sold to third parties.
Outlook.com uses SmartScreen to protect users from unsolicited e-mail messages (spam), fraudulent emails (phishing) and malware spread via e-mail. After its initial review of the body text, the system focuses on the hyperlinks and attachments.
Junk mail (spam)
To filter spam, SmartScreen Filter uses machine learning from Microsoft Research which learns from known spam threats and user feedback when emails are marked as "Spam" by the user.
Over time, these preferences help SmartScreen Filter to distinguish between the characteristics of unwanted and legitimate e-mail and can also determine the reputation of senders by a number of emails having had this checked. Using these algorithms and the reputation of the sender is an SCL rating (Spam Confidence Level score) assigned to each e-mail message (the lower the score, the more desirable). A score of -1, 0, or 1 is considered not spam, and the message is delivered to the recipient's inbox. A score of 5, 6, 7, 8, or 9 is considered spam and is delivered to the recipient's Junk Folder. Scores of 5 or 6 are considered to be suspected spam, while a score of 9 is considered certainly spam. The SCL score of an email can be found in the various x-headers of the received email.
SmartScreen Filter also analyses email messages from fraudulent and suspicious Web links. If such suspicious characteristics are found in an email, the message is either[clarification needed] directly sent to the Spam folder with a red information bar at the top of the message which warns of the suspect properties. SmartScreen also protects against spoofed domain names (spoofing) in emails to verify whether an email is sent by the domain which it claims to be sent. For this, it uses the technology Sender ID and DomainKeys Identified Mail (DKIM). SmartScreen Filter also ensures that one email[clarification needed] from authenticated senders can distinguish more easily by placing a green-shield icon for the subject line of these emails.
In other products
Code Signing Certificates
SmartScreen builds reputation based on code signing certificates that identify the author of the software. This means that once a reputation has been built, new versions of an application can be signed with the same certificate and maintain the same reputation.
However, code signing certificates need to be renewed every two years. SmartScreen does not relate a renewed certificate to an expired one. This means that reputations need to be rebuilt every two years, with users getting frightening messages in the meantime. Extended Validation (EV) certificates seem to avoid this issue, but they are expensive and difficult to obtain for small developers. 
In late 2010, the results of browser malware testing undertaken by NSS Labs were published. The study looked at the browser's capability to prevent users following socially engineered links of a malicious nature and downloading malicious software. It did not test the browser's ability to block malicious web pages or code.
According to NSS Labs, Internet Explorer 9 blocked 99% of malware downloads compared to 90% for Internet Explorer 8 that does not have the SmartScreen Application Reputation feature as opposed to the 13% achieved by Firefox, Chrome, and Safari; which all use a Google Safe Browsing malware filter. Opera 11 was found to block just 5% of malware. SmartScreen Filter was also noted for adding legitimate sites to its blocklists almost instantaneously, as opposed to the several hours it took for blocklists to be updated on other browsers.
In early 2010, similar tests had given Internet Explorer 8 an 85% passing grade, the 5% improvement being attributed to "continued investments in improved data intelligence". By comparison, the same research showed that Chrome 6, Firefox 3.6 and Safari 5 scored 6%, 19% and 11%, respectively. Opera 10 scored 0%, failing to "detect any of the socially engineered malware samples".
Manufacturers of other browsers criticized the test, focusing upon the lack of transparency of URLs tested and the lack of consideration of layered security additional to the browser, with Google commenting that "The report itself clearly states that it does not evaluate browser security related to vulnerabilities in plug-ins or the browsers themselves", and Opera commenting that the results appeared "odd that they received no results from our data providers" and that "social malware protection is not an indicator of overall browser security".
In July 2010, Microsoft claimed that SmartScreen on Internet Explorer had blocked over a billion attempts to access sites containing security risks. According to Microsoft, the SmartScreen Filter included in Outlook.com blocks 4.5 billion unwanted e-mails daily from reaching users. Microsoft also claims that only 3% of incoming email is junk mail but a test by Cascade Insights says that just under half of all junk mail still arrives in the inbox of users. In a September 2011 blog post, Microsoft stated that 1.5 billion attempted malware attacks and over 150 million attempted phishing attacks have been stopped.
In October 2017, criticisms regarding URL submission methods were addressed with the creation of the Report unsafe site URL submission page. Microsoft now supports URL submissions from this form as opposed to the previous experience of a user having to visit the site and use in-product reporting methods.
SmartScreen Filter can be bypassed. Some phishing attacks use a phishing email linking to a front-end URL not in the Microsoft database; clicking this URL in the email redirects the user to the malicious site. The "report this website" option in Internet Explorer only reports the currently-open page; the front-end URL in the phishing attack cannot be reported to Microsoft and remains accessible.
SmartScreen Filter creates a problem for small software vendors when they distribute an updated version of installation or binary files over the internet. Whenever an updated version is released, SmartScreen responds by stating that the file is not commonly downloaded and can therefore install harmful files on your system. This can be fixed by the author digitally signing the distributed software. Reputation is then based not only on a file's hash but on the signing certificate as well. A common distribution method for authors to bypass SmartScreen warnings is to pack their installation program (for example Setup.exe) into a ZIP-archive and distribute it that way, though this can confuse novice users.
Another criticism is that SmartScreen increases the cost of makes non-commercial and small scale software development. Developers either have to purchase standard code signing certificates or more expensive extended validation certificates. Extended validation certificates allow the developer to immediately establish reputation with SmartScreen  but are often unaffordable for people developing software either for free or not for immediate profit. The standard code signing certicates however pose a "catch-22" for developers, since SmartScreen warnings make people reluctant to download software, as a consequence to get downloads requires first passing Smartscreen, passing SmartScreen requires getting reputation and getting reputation is dependent on downloads.
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