HTTP Strict Transport Security
HTTP Strict Transport Security (HSTS) is a simple and widely supported standard to protect visitors by ensuring that their browsers always connect to a website over HTTPS. HSTS exists to remove the need for the common, insecure practice of redirecting users from
When a browser knows that a domain has enabled HSTS, it does two things:
- Always uses an
https://connection, even when clicking on an
http://link or after typing a domain into the location bar without specifying a protocol.
- Removes the ability for users to click through warnings about invalid certificates.
A domain instructs browsers that it has enabled HSTS by returning an HTTP header over an HTTPS connection.
In its simplest form, the policy tells a browser to enable HSTS for that exact domain or subdomain, and to remember it for a given number of seconds:
In its strongest and recommended form, the HSTS policy includes all subdomains, and indicates a willingness to be “preloaded” into browsers:
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
When using this form, bear in mind:
- The policy should be deployed at
- All subdomains associated with the parent domain must support HTTPS. (They do not have to each have their own HSTS policy.)
See below for examples of how to set an HSTS policy in common web servers.
Strict Transport Security was proposed in 2009, motivated by Moxie Marlinspike’s demonstration of how a hostile network could downgrade visitor connections and exploit insecure redirects. It was quickly adopted by several major web browsers, and finalized as RFC 6797 in 2012.
The basic problem that HSTS solves is that even after a website turns on HTTPS, visitors may still end up trying to connect over plain HTTP. For example:
- When a user types “dccode.gov” into the URL bar, browsers default to using
- A user may click on an old link that mistakenly uses an
- A user’s network may be hostile and actively rewrite
Websites that prefer HTTPS will generally still listen for connections over HTTP in order to redirect the user to the HTTPS URL. For example:
$ curl --head http://www.facebook.com HTTP/1.1 301 Moved Permanently Location: https://www.facebook.com/
This redirect is insecure and is an opportunity for an attacker to capture information about the visitor (such as cookies from a previous secure session), or to maliciously redirect the user to a phishing site.
This can be addressed by returning a
Strict-Transport-Security header whenever the user connects securely. For example:
$ curl --head https://www.facebook.com HTTP/1.1 200 OK Strict-Transport-Security: max-age=15552000; preload
This enables HSTS for
www.facebook.com. While HSTS is in effect, clicking any links to
http://www.facebook.com will cause the browser to issue a request directly for
In the above example, the browser will remember the HSTS policy for 180 days. The policy is refreshed every time browser sees the header again, so if a user visits
https://www.facebook.com at least once every 180 days, they’ll be indefinitely protected by HSTS.
For a user to take advantage of HSTS, their browser does have to see the HSTS header at least once. This means that users are not protected until after their first successful secure connection to a given domain.
In addition, in many cases, there may never be a first visit to
https://domain.gov. For example:
- Many federal websites redirect directly from
- Many federal domains that are used solely for redirects will redirect from
In either case,
https://domain.gov is never visited, meaning connecting clients will never see an HSTS policy with an
includeSubDomains directive that applies to the whole zone.
To solve this problem, the Chrome security team created an “HSTS preload list”: a list of domains baked into Chrome that get Strict Transport Security enabled automatically, even for the first visit.
Firefox, Safari, Opera, and Edge also incorporate Chrome’s HSTS preload list, making this feature shared across major browsers.
How to preload a domain
The Chrome security team allows anyone to submit their domain to the list, provided it meets the following requirements:
- HTTPS is enabled on the root domain (e.g.
https://donotcall.gov), and all subdomains (e.g.
https://www.donotcall.gov) – especially the
wwwsubdomain, if a DNS record for it exists. This necessarily includes any subdomains in use solely on intranets.
- The HSTS policy includes all subdomains, with a long
max-age, and a
preloadflag to indicate that the domain owner consents to preloading.
- The website redirects from HTTP to HTTPS, at least on the root domain.
An example of a valid HSTS header for preloading:
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
In the long term, as the web transitions fully to HTTPS and browsers can start phasing out plain HTTP and defaulting to HTTPS, the HSTS preload list (and HSTS itself) may eventually become unnecessary.
Until that time, the HSTS preload list is a simple, effective mechanism for locking down HTTPS for an entire domain.
HSTS as a forcing function
Strict Transport Security provides meaningful security benefits to visitors, especially visitors on hostile networks.
However, it’s also highly valuable as an organizational forcing function and compliance mechanism.
When a domain owner follows the recommendations in this article and sets an HSTS policy on its base domain with
preload, the domain owner is saying “Every part of our web infrastructure is HTTPS, and always will be.” — and is giving browsers permission to vigorously enforce that from then onwards.
It’s a clear and auditable commitment, and gives anyone overseeing an organization’s transition to HTTPS a way of marking domains as “done”.
Zooming out even further: it’s technically possible to preload HSTS for an entire top-level domain (e.g. “.gov”), as Google first did with
.gov can get there.
Configuration for common web servers
On nginx, you would apply an
add_header command to the appropriate virtual host configuration. This website,
https.cio.gov, is hosted on nginx, and uses this batch of HTTPS rules to set this header:
add_header Strict-Transport-Security 'max-age=31536000; includeSubDomains; preload;' always;
On Apache, you would apply a
Header directive to always set the HSTS header, like so:
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
On Microsoft systems running IIS (Internet Information Services), there are no “.htaccess” files to implement custom headers. IIS applications use a central “web.config” file for configuration. For IIS 7.0 and up, the code is as follows:
<httpProtocol> <customHeaders> <add name="Strict-Transport-Security" value="max-age=31536000; includeSubDomains; preload "/> </customHeaders> </httpProtocol>
Generally, you want to set a custom HTTP header for
Strict-Transport-Security with the value
max-age=31536000; includeSubDomains; preload (or some variant).
Here are some links to do that with other web servers:
- Browser support for HSTS
- HSTS web developer documentation maintained by the Mozilla community
- Chrome’s HSTS preload list, and their submission form.
- “Upgrading HTTPS in Mid-Air” - A paper analyzing the current detailed practice of HSTS and HTTP Public Key Pinning, as of November 2014.
- “The first .gov domains hardcoded into your browser as all-HTTPS”, by 18F.