HTTP Strict Transport Security

Description
HTTP Strict Transport Security (HSTS) is an opt-in security enhancement that is specified by a web application through the use of a special response header. Once a supported browser receives this header that browser will prevent any communications from being sent over HTTP to the specified domain and will instead send all communications over HTTPS. It also prevents HTTPS click through prompts on browsers.

Examples
Example of the HTTP strict transport security header

Strict-Transport-Security: max-age=60000

If all subdomains are HTTPS to then the following header is applicable:

Strict-Transport-Security: max-age=60000; includeSubDomains

Server Side
The web server side needs to inject the HSTS header.

For HTTP sites on the same domain it is not recommended to add a HSTS header but to do a permanent redirect (301 status code) to the HTTPS site. An Apache HTTPd example that will permanently redirect a URL to the identical URL with a HTTPS scheme, is as follows:

 ServerAlias * RewriteEngine On       RewriteRule ^(.*)$ https://%{HTTP_HOST}$1 [redirect=301] 

On the HTTPS site configuration the following is needed to add the header as recommended by the standard: Header set Strict-Transport-Security "max-age=16070400; includeSubDomains"

The following links show how to do set response headers in other web servers:
 * IIS
 * NGINX
 * Lighttpd
 * HTTPd

Threats
HSTS addresses the following threats:
 * User bookmarks or manually types http://example.com and is subject to a man-in-the-middle attacker
 * HSTS automatically upgrades HTTP requests to HTTPS for the target domain
 * Web application that is intended to be purely HTTPS inadvertently contains HTTP links or serves content over HTTP
 * HSTS automatically upgrades HTTP requests to HTTPS for the target domain
 * A man-in-the-middle attacker attempts to intercept traffic from a victim user using an invalid certificate and hopes the user will accept the bad certificate
 * HSTS does not allow a user to override the invalid certificate message

Links
Chromium Projects/HSTS

HSTS Spec

Wikipedia

Mozilla Developer Network

OWASP TLS Protection Cheat Sheet

Firefox STS Support

Google Chrome STS Support