HTTP Strict Transport Security

HTTP Strict Transport Security (often abbreviated as HSTS) is a security feature that lets a web site tell browsers that it should only be communicated with using HTTPS, instead of using HTTP.

How it helps

If a web site accepts a connection through HTTP and redirects to HTTPS, the user in this case may initially talk to the non-encrypted version of the site before being redirected, if, for example, the user types https://www.foo.com/ or even just foo.com.

This opens up the potential for a man-in-the-middle attack, where the redirect could be exploited to direct a user to a malicious site instead of the secure version of the original page.

The HTTP Strict Transport Security feature lets a web site inform the browser that it should never load the 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.

Enabling Strict Transport Security

Enabling this feature for your site is as simple as returning the Strict-Transport-Security HTTP header when your site is accessed over HTTPS:

Strict-Transport-Security: max-age=expireTime [; includeSubDomains] [; preload]

max-age

The time, in seconds, that the browser should remember that this 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.

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 to use (or start using) the submitted domains.

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

Specifications

Browser compatibility

[1] Added with KB 3058515 in June 2015

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