HSTS restricts the connection to be always HTTPS if deployed by any domain, however for it to be applied to sub-domains the 'includeSubDomain' attribute is needed. Why doesn't the policy itself make it mandatory to include all subdomains? For what reasons/restrictions is the flag asked to be set?

  • Wouldn't that require certificates for each sub domain or a wildcard certificate? You cannot use HSTS with self signed certificates. This would lead to significantly higher deployment costs and thus hurt the adoption of HSTS. – Max Ried Oct 29 '16 at 8:58

Subdomains can often be used for different purposes, and as a result they can be using different web applications possibly hosted on different equipment.

Not every website on every subdomain needs to follow the policy of the domain, it can even be the case that sites on subdomains do not support HTTPS due to lack of support in the application and/or not having a SSL certificate.

So automatically enforcing a policy on the main domain could possibly break things for subdomains.

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    +1 - if HSTS was subdomain mandatory, many orgs could not use it if they have an Active Directory domain which is a subdomain off their public domain. Which is exactly what Microsoft publish as best practice. – Mark Henderson Oct 28 '16 at 15:18
  • @MarkHenderson why would that affect AD? – Robert Grant Oct 28 '16 at 18:18
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    @RobertGrant say you own example.com and you deploy your AD on ad.example.com - now you set up a web service at service.ad.example.com - if HSTS was forced on for subdomains, it would be forced on for service.ad.example.com which you may not want, or may not even be supported for that application/appliance. – Mark Henderson Oct 28 '16 at 19:03
  • Something else that comes to my mind is to serve cookie-less static content. This is usually done by having your site example.com fetch images on http://images.example.com/, instead of httpS://example.com/images/, which can improve on speed and parallel download of files. – Ismael Miguel Oct 28 '16 at 22:21
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    @IsmaelMiguel having images over HTTP in a HTTPS page is a bad idea. I think most browsers would at least give a mixed-content warning. – Paŭlo Ebermann Oct 28 '16 at 22:36

It's best to let the site owner decide whether subdomains are affected, just as the site owner decides whether to use HSTS at all. The extra flexibility could help improve HSTS adoption by reducing compatibility obstacles.

Some of the subdomain services may not have an HTTPS server installed yet. There are a few cases where it's not important security-wise to have HTTPS on certain subdomains.

That answers why includeSubDomain is not mandatory.

One could still ask why it is not the default:

For this I would like to point out that HSTS cannot be undone except by asking your visitors to clear cache in their own browser. Chances are your visitors will not do this.

So, if you accidentally applied HSTS to more than you intended to, that (sub)domain goes down permanently for those visitors until they switch devices or browsers, clear cache on the browser, or the site owner installs an HTTPS server. (which might be technically difficult on some legacy services)

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    Chances are at that point you have no easy way of contacting the visitors of your site in the first place. – Michael Kjörling Oct 28 '16 at 14:07
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    "Some of the subdomain services may not have an HTTPS server installed yet." -- An in fact, may not even be under your control. It's fairly common to add DNS records for subdomains for SaaS apps like a CRM, recruiting software, etc. – Xiong Chiamiov Oct 28 '16 at 19:26

My best example on why this would be bad, is webhosting. Imagine a webhost, lets say somecheapwebhostingexample.com has HSTS and even a payment page. And then they don't have wildcard certificates or anything like that.

Then a customer signs up as customer1.somecheapwebhostingexample.com . Its obvious its a bad example to deploy subdomains per default here.

There are other examples where theres multiple subdomains, but no certificate available for them all.

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