With today’s monthly security updates (KB 3058515), Microsoft is bringing the protections offered by HSTS to Internet Explorer 11 on Windows 8.1 and Windows 7. HSTS is also available in both Internet Explorer 11 and Microsoft Edge on Windows 10.
The HTTP Strict Transport Security (HSTS) policy protects against variants of man-in-the-middle attacks that can strip TLS out of communications with a server, leaving the user vulnerable.
Site developers can use HSTS policies to secure connections by opting in to an HSTS preload list, which registers websites to be hardcoded by Microsoft Edge, Internet Explorer, and other browsers to redirect HTTP traffic to HTTPS. Communications with these websites from the initial connection are automatically upgraded to be secure. Like other browsers which have implemented this feature, Microsoft Edge and Internet Explorer 11 base their preload list on the Chromium HSTS preload list.
Alternatively, sites not on the preload list can enable HSTS via the Strict-Transport-Security HTTP header. After an initial HTTPS connection from the client containing the HSTS header, any subsequent HTTP connections are redirected by the browser to be secured via HTTPS.
As a note, in Microsoft Edge on Windows 10, mixed content is always blocked on servers supporting HSTS. For Internet Explorer 11 on Windows 10, Windows 8.1, and Windows 7, the Information bar will prompt the user to proceed in mixed content scenarios.
In addition to the HSTS updates, this month’s Internet Explorer updates include security fixes.