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.

The specification has been released and published end of 2012 as RFC 6797 (HTTP Strict Transport Security (HSTS)) by the IETF. (Reference see in the links at the bottom.)

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 redirects 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 redirects 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

Examples
Simple example, using a long (1 year) max-age:

Strict-Transport-Security: max-age=31536000

If all present and future subdomains will be HTTPS:

Strict-Transport-Security: max-age=31536000; includeSubDomains

Recommended: If the site owner would like their domain to be included in the HSTS preload list maintained by Chrome (and used by Firefox and Safari), then use:

Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

The `preload` flag indicates the site owner's consent to have their domain preloaded. The site owner still needs to then go and submit the domain to the list.

Problems
Site owners can use HSTS to identify users without cookies. This can lead to a significant privacy leak.

Cookies can be manipulated from sub-domains, so omitting the include "includeSubDomains" option permits a broad range of cookie-related attacks that HSTS would otherwise prevents by requiring a valid certificate for a subdomain. Ensuring the "Secure Flag" is set on all cookies will also prevent, some, but not all, of the same attacks.

Browser Support
A detailed overview of supporting browsers can be found at caniuse.com. There is also a TLS Browser Test Page to check whether your current browser supports HSTS.

Links

 * AppSecTutorial Series - Episode 4
 * Chromium Projects/HSTS
 * HSTS Spec
 * Wikipedia
 * Mozilla Developer Network
 * OWASP TLS Protection Cheat Sheet
 * Firefox STS Support
 * Google Chrome STS Support
 * Moxie Marlinspike's Black Hat 2009 talk on sslstrip, that demonstrates why you need HSTS