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HSTS in IIS

In our continuous effort to improve the security of the solutions we build, we activated HSTS at one of our clients. However I noticed that there was a lack of understanding on what HSTS exactly is and how it helps to improve security. Hence this blog post.

What is HSTS?

Let’s ask Wikipedia:

HTTP Strict Transport Security (HSTS) is a policy mechanism that helps to protect websites against man-in-the-middle attacks such as protocol downgrade attacks and cookie hijacking. It allows web servers to declare that web browsers (or other complying user agents) should automatically interact with it using only HTTPS connections.

In short HSTS tells a browser to only access a site through HTTPS.

Enabling HSTS in IIS

Let me first show you to enable it in IIS before I get into more detail.

  • Open IIS (InetMgr)

  • Click on the website where you want to activate HSTS.
    • Remark: HSTS is always activated at the site level.

    • On the Manage Website section, click on HSTS…
    • On the Edit Website HSTS they are multiple values we can configure after checking the Enable checkbox:
      • Max-Age: The amount of time in seconds that this website will be served over HTTPS
      • IncludeSubDomains: Specify if HSTS should apply to all subdomains as well
      • Preload: It is possible to make your domain part of a preload list. By doing this you are sure that browsers will connect to your domain only via secure connections. (This is not directly part of the HSTS specification and only applies when Max-Age is at least 1 year and IncludeSubDomains is set to true)
      • Redirect Http to Https: Enable this to let IIS redirect from HTTP to HTTPS when visiting this site

    More information: IIS 10.0 Version 1709 HTTP Strict Transport Security (HSTS) Support | Microsoft Learn

    What HSTS doesn’t do…

    After activating HSTS I noticed that there was some confusion on what HSTS exactly does and how it works.

    If you visit a website using HTTP, no HSTS response header is returned and nothing happens. It is only AFTER you have visited the website using HTTPS that HSTS will kick in. I really want to emphasize this because I noticed that this was not what some people expected.

    This means that either the user has directly browsed to the website using HTTPS or he/she is redirect to the HTTPS version. This can be done for example through the URL Rewrite Module in IIS or by selecting the ‘Redirect Http to Https’ checkbox as mentioned above.

    Now the browser will remember for the max-age specified that this site should be served from HTTPS. So if you attempt to browse to the HTTP endpoint, the browser itself will rewrite the url to use the HTTPS endpoint and no connection is made over HTTP.

    Also be aware that HSTS won’t let you click through certificate warnings. So, if the certificate is not correct, HSTS won’t work.

    Hope that helps…

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