Making an Azure static website EVEN MORE secure

Remember how I was congratulating myself that I’d made my jmbucknall.com static website, that is hosted on Azure, secure? How I’d bought and uploaded an SSL certificate, and made the site only accessible via HTTPS? Well, HA!

I say that, because Barry Dorrans (self-described as “Microsoft's .NET security person”) was ‘kind’ enough to point out that I hadn’t really finished the job. I hadn’t added the proper “security headers” (WTF are they?) via a web.config (wut? it’s a static site!) and that I should go immediately to securityheaders.io to scan my site. Which I did, and got a resounding F. Um, gee thanks, Barry.

So, yesterday, it was time to investigate and to fix and to get an A+.

I’ll admit that a lot of this was brand new to me, but let’s start from the beginning. When you visit a website, your browser builds a request and sends it to the web server that’s serving up the content (HTML, CSS, script files, images, whatever). That request has a set of request headers that describe what the browser can deal with and what it is expecting (things like the type of files, the language, any encodings, and so  on).

Request headers

Example request headers

The web server, in replying, does the same: a chunk of data (HTML, CSS, etc) with a set of response headers. The security headers Barry pointed me to form part of these response headers. Whether the site is static or not, these headers are provided by the web server, not the site, and it is through the web.config file that we can insert new headers or take out the ones we don’t want to send back.

Example unsecure response headers

Unsecure response headers

So, despite my conceit that a static website has no need for a web.config, in fact it does. Think of it in this case as a set of instructions to the web server, rather than being part of the site’s static content.

(Aside: when I started to gather the code for the security headers, I discovered that, in fact, I already had a web.config for jmbucknall.com. I’d completely forgotten I’d added it, and had overlooked it since then. Its only purpose was to declare the MIME type for the .woff font files I use in the site; in other words an instruction to the web server that .woff files are valid.)

Next, then, was to work out which security headers to add and how to define them in my web.config for the site. As it turns out, securityheaders.io makes that easy: it scores you according to which headers your server is returning or not. I was … not, to be blunt.

Security Report

Example security report

Yep, six security headers are missing. Luckily the scan site gives you a quick description for each (which I’ve quoted extensively here).

Strict-Transport-Security

HTTP Strict Transport Security (HSTS) is an excellent feature to support on your site and strengthens your implementation of TLS by getting the User Agent to enforce the use of HTTPS. Recommended value "strict-transport-security: max-age=31536000; includeSubDomains".

To add this to the config file involves a weird-looking Redirect rule (to force a redirect of HTTP to HTTPS) and an equally weird-looking Rewrite rule (once you’ve used HTTPS, always use it). The best discussion of this I found is from Scott Hanselman’s blog a couple of years back.

<configuration>
  <system.webServer>
    <rewrite>
      <rules>
        <rule name="HTTP to HTTPS redirect" stopProcessing="true">
          <match url="(.*)" />
          <conditions>
              <add input="{HTTPS}" pattern="off" ignoreCase="true" />
          </conditions>
          <action type="Redirect" url="https://{HTTP_HOST}/{R:1}" redirectType="Permanent" />
        </rule>
      </rules>    
      <outboundRules>
        <rule name="Add Strict-Transport-Security when HTTPS" enabled="true">
          <match serverVariable="RESPONSE_Strict_Transport_Security" pattern=".*" />
          <conditions>
            <add input="{HTTPS}" pattern="on" ignoreCase="true" />
          </conditions>
          <action type="Rewrite" value="max-age=31536000; includeSubdomains; preload" />
        </rule>
... </outboundRules> </rewrite>
... </system.webServer> </configuration>

To be honest, I think that the Redirect is actually already done by setting that HTTPS Only option in the Azure dashboard. One day, when I have a spare 5 minutes, I’ll experiment.

Content-Security-Policy

Content Security Policy (CSP) is an effective measure to protect your site from XSS attacks. By whitelisting sources of approved content, you can prevent the browser from loading malicious assets.

This one is again done through a Rewrite rule, but it involves a whole bunch of extra work in the website’s content files as I found out. I’ll talk about this a bit later.

<configuration>
  <system.webServer>
    <rewrite>
      <outboundRules>
<rule name="CSP"> <match serverVariable="RESPONSE_Content-Security-Policy" pattern=".*" /> <action type="Rewrite" value="default-src 'self'; script-src 'self' https://code.jquery.com https://ssl.google-analytics.com; img-src 'self' https://ssl.google-analytics.com; style-src 'self' https://maxcdn.bootstrapcdn.com; font-src 'self' https://maxcdn.bootstrapcdn.com;" /> </rule> </outboundRules> </rewrite>
</system.webServer> </configuration>

Yes, this rule does have a horrendously long value for the action. More on that in a moment.

The next four headers are simple to set as a block.

X-Frame Options

X-Frame-Options tells the browser whether you want to allow your site to be framed or not. By preventing a browser from framing your site you can defend against attacks like clickjacking. Recommended value "x-frame-options: SAMEORIGIN".

X-XSS-Protection

X-XSS-Protection sets the configuration for the cross-site scripting filter built into most browsers. Recommended value "X-XSS-Protection: 1; mode=block".

X-Content-Type-Options

X-Content-Type-Options stops a browser from trying to MIME-sniff the content type and forces it to stick with the declared content-type. The only valid value for this header is "X-Content-Type-Options: nosniff".

Referrer Policy

Referrer Policy is a new header that allows a site to control how much information the browser includes with navigations away from a document and should be set by all sites.

The code for these four looks like this:

<configuration>
  <system.webServer>
<httpProtocol> <customHeaders> <clear /> <!-- Gets rid of the other unwanted headers --> <add name="X-Frame-Options" value="SAMEORIGIN" /> <add name="X-Xss-Protection" value="1; mode=block" /> <add name="X-Content-Type-Options" value="nosniff" /> <add name="Referrer-Policy" value="no-referrer" /> </customHeaders> <redirectHeaders> <clear /> </redirectHeaders> </httpProtocol> </system.webServer> </configuration>

Back to the CSP

If you look back at the CSP code, you’ll see that it has a value for the action that is in essence a set of clauses separated by semi-colons. Each of those clauses defines where a particular content type can be downloaded from. So, for example, the script-src clause defines from where the browser is allowed to download scripts: there’s 'self' meaning the current domain, then there’s one url for jQuery, and one for Google analytics. The other clauses are all pretty self-explanatory. The header is basically saying: ONLY download this particular type of content from these urls. No others.

Of course, when I wrote the content for jmbucknall.com, I just wrote the markup, etc, as I’d always done. Didn’t have to whitelist anything. It just worked. Now, in this new security landscape, I have to, in order to avoid XSS-type attacks. So, what happened was that, once I’d uploaded my new web.config, suddenly, parts of my site no longer rendered properly. I had to go through and fix all those references to “outside” urls.

The other fun bit I had to fix was the Google Analytics code. Be default (at least when I wrote the HTML), it was a bunch of script elements on the page, plus it did a document.write to add some more. Under the CSP, inline scripts are not allowed. (Strictly speaking, they are, but you have to add an option whose very name should strike terror into your secure heart: 'unsafe-inline'.) So, I took those inline scripts out of the HTML and put them in a JS file. But, hey, I don’t use inline scripts in my own markup, never have done (they’re cheesy and cheating), so that was easy.

Secure response headers

Secure response headers

And there we have it, Part Deux on making your Azure-hosted static website secure. Now I’ve done it once, time to experiment with another of my domains, this time with a Let’s Encrypt certificate. Because it’s free or something.

Secure site banner

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