# Use `Strict-Transport-Security` header (`strict-transport-security`)
`strict-transport-security` warns against serving resources over
HTTPS without `strict-transport-security` header and validates the
header directives and their corresponding values.
## Why is this important?
Web security should be a critical concern for web developers.
Unlike [cross-site scripting (XSS)][xss] and [SQL injection][sql
injection], the exploit of insufficient protection over the
transport layer can be harder to picture in practice. If a website
accepts a connection through HTTP and then redirects to HTTPS, it
opens opportunities for a "man-in-the-middle" attack, when the
redirect could be exploited and lead the user to a malicious site.
By specifying the `Strict-Transport-Security` header along with
a `max-age` value in the response, a website can declare that only
secure connections within the specified period will be accepted.
For future requests to the same domain via insecure connections,
the browser knows that it should never load the site using HTTP
and automatically converts all requests to HTTPS instead.
Notably, to prevent the `Strict-Transport-Security` header from being
stripped by the attacker on the user’s first visit, major browsers
include a "pre-loaded" list of sites that must be loaded via HTTPS.
You can submit your domain name in the [online form][preload form]
to be included in the list. After being included, all insecure
connection requests will be disallowed. Use with great caution:
Before you decide to have your own domain included, make sure that
you can support HTTPS for all the subdomains and that you'll never
again need the insecure scheme.
More information about HTTP Strict Transport (HSTS), please see:
* [HTTP Strict Transport Security wiki][hsts wiki]
* [HTTP Strict Transport Security Cheat Sheet][hsts cheat sheat]
## What does the hint check?
For a site served over HTTPS, this hint checks the following:
* If it has a `Strict-Transport-Security` header.
* If the header has the required `max-age` directive.
* If the `max-age` directive has a value that is longer than
18 weeks (10886400s).
* If `Strict-Transport-Security` header has repetitive directives.
* When a `Strict-Transport-Security` header contains the `preload`
directive, this hint will first check the domain name against the
[HTTP Strict Transport Security (HSTS) preload list][preload list]
for the preload status, and then check whether this domain has
errors that would prevent preloading by calling the HSTS Preload
API endpoint. This check is disabled by default.
### Examples that **trigger** the hint
`Strict-Transport-Security` response header was not sent over `HTTPS`:
```text
HTTP/... 200 OK
...
```
`Strict-Transport-Security` response header is sent with a `max-age`
value that is too short:
```text
HTTP/... 200 OK
...
Strict-Transport-Security: max-age=1
```
`Strict-Transport-Security` response header is sent without `max-age`
directive:
```text
HTTP/... 200 OK
...
Strict-Transport-Security: maxage=31536000
```
`Strict-Transport-Security` response header is sent with duplicate
`includeSubDomains` directives:
```text
HTTP/... 200 OK
...
Strict-Transport-Security: includeSubDomains; max-age=31536000; includeSubDomains
```
### Examples that **pass** the hint
```text
HTTP/... 200 OK
...
Strict-Transport-Security: max-age=31536000
```
```text
HTTP/... 200 OK
...
Strict-Transport-Security: max-age=31536000; includeSubDomains
```
```text
HTTP/... 200 OK
...
Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
```
## How to configure the server to pass this hint
How to configure Apache
Apache can be configured to serve resources with the
`Strict-Transport-Security` header with a specific value
using the [`Header` directive][header directive], e.g.:
```apache
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
```
Note that:
* The above snippet works with Apache `v2.2.0+`, but you need to have
[`mod_headers`][mod_headers] [enabled][how to enable apache modules]
in order for it to take effect.
* If you have access to the [main Apache configuration file][main
apache conf file] (usually called `httpd.conf`), you should add
the logic in, for example, a [``][apache directory]
section in that file. This is usually the recommended way as
[using `.htaccess` files slows down][htaccess is slow] Apache!
If you don't have access to the main configuration file (quite
common with hosting services), add the snippets in a `.htaccess`
file in the root of the web site/app.
For the complete set of configurations, not just for this rule, see
the [Apache server configuration related documentation][apache config].
How to configure IIS
IIS can be configured to serve resources with the `Strict-Transport-Security`
header with a specific value using the [` element`][customHeader].
E.g.:
```xml
```
Note that:
* The above snippet works with IIS 7+.
* You should use the above snippet in the `web.config` of your
application.
For the complete set of configurations, not just for this rule,
see the [IIS server configuration related documentation][iis config].