Strict-Transport-Security

The Strict-Transport-Security response header (often abbreviated as HSTS) lets a web site tell browsers that it should only be accessed using HTTPS, instead of using HTTP.

Syntax

Strict-Transport-Security: max-age=<expire-time>
Strict-Transport-Security: max-age=<expire-time>; includeSubDomains
Strict-Transport-Security: max-age=<expire-time>; preload

Directives

max-age=<expire-time>

The time, in seconds, that the browser should remember that a site is only to be accessed using HTTPS.

includeSubDomains Optional

If this optional parameter is specified, this rule applies to all of the site's subdomains as well.

preload Optional

See Preloading Strict Transport Security for details. Not part of the specification.

Description

If a website accepts a connection through HTTP and redirects to HTTPS, visitors may initially communicate with the non-encrypted version of the site before being redirected, if, for example, the visitor types http://www.foo.com/ or even just foo.com. This creates an opportunity for a man-in-the-middle attack. The redirect could be exploited to direct visitors to a malicious site instead of the secure version of the original site.

The HTTP Strict Transport Security header informs the browser that it should never load a site using HTTP and should automatically convert all attempts to access the site using HTTP to HTTPS requests instead.

An example scenario

You log into a free WiFi access point at an airport and start surfing the web, visiting your online banking service to check your balance and pay a couple of bills. Unfortunately, the access point you're using is actually a hacker's laptop, and they're intercepting your original HTTP request and redirecting you to a clone of your bank's site instead of the real thing. Now your private data is exposed to the hacker.

Strict Transport Security resolves this problem; as long as you've accessed your bank's web site once using HTTPS, and the bank's web site uses Strict Transport Security, your browser will know to automatically use only HTTPS, which prevents hackers from performing this sort of man-in-the-middle attack.

How the browser handles it

The first time your site is accessed using HTTPS and it returns the Strict-Transport-Security header, the browser records this information, so that future attempts to load the site using HTTP will automatically use HTTPS instead.

When the expiration time specified by the Strict-Transport-Security header elapses, the next attempt to load the site via HTTP will proceed as normal instead of automatically using HTTPS.

Whenever the Strict-Transport-Security header is delivered to the browser, it will update the expiration time for that site, so sites can refresh this information and prevent the timeout from expiring. Should it be necessary to disable Strict Transport Security, setting the max-age to 0 (over a https connection) will immediately expire the Strict-Transport-Security header, allowing access via http.

Preloading Strict Transport Security

Google maintains an HSTS preload service. By following the guidelines and successfully submitting your domain, browsers will never connect to your domain using an insecure connection. While the service is hosted by Google, all browsers have stated an intent to use (or actually started using) the preload list. However, it is not part of the HSTS specification and should not be treated as official.

• Information regarding the HSTS preload list in Chrome : https://www.chromium.org/hsts
• Consultation of the Firefox HSTS preload list : nsSTSPreloadList.inc

Examples

All present and future subdomains will be HTTPS for a max-age of 1 year. This blocks access to pages or sub domains that can only be served over HTTP.

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

Specifications

Browser compatibilityUpdate compatibility data on GitHub

See also

• Blog post: HTTP Strict Transport Security has landed!
• Blog post: HTTP Strict Transport Security (force HTTPS)
• OWASP Article: HTTP Strict Transport Security
• Wikipedia: HTTP Strict Transport Security
• Browser test site: HSTS and HPKP test