Adding third-party embedded widgets on a website is a common but potentially dangerous practice. Thankfully, the web platform offers a few controls that can help mitigate the risks. While this post uses the example of an embedded SurveyMonkey survey, the principles can be used for all kinds of other widgets.

Note that this is by no means an endorsement of SurveyMonkey's proprietary service. If you are looking for a survey product, you should consider a free and open source alternative like LimeSurvey.

SurveyMonkey's snippet

In order to embed a survey on your website, the SurveyMonkey interface will tell you to install the following website collector script:

<script>(function(t,e,s,n){var
o,a,c;t.SMCX=t.SMCX||[],e.getElementById(n)||(o=e.getElementsByTagName(s),a=o[o.length-1],c=e.createElement(s),c.type="text/javascript",c.async=!0,c.id=n,c.src=["https:"===location.protocol?"https://":"http://","widget.surveymonkey.com/collect/website/js/tRaiETqnLgj758hTBazgd9NxKf_2BhnTfDFrN34n_2BjT1Kk0sqrObugJL8ZXdb_2BaREa.js"].join(""),a.parentNode.insertBefore(c,a))})(window,document,"script","smcx-sdk");</script><a
style="font: 12px Helvetica, sans-serif; color: #999; text-decoration:
none;" href=https://www.surveymonkey.com> Create your own user feedback
survey </a>

which can be rewritten in a more understandable form as:

(
function (s) {
  var scripts, last_script, new_script;
  window.SMCX = window.SMCX || [],
  document.getElementById("smcx-sdk") ||
    (
      scripts = document.getElementsByTagName("script"),
      last_script = scripts[scripts.length - 1],
      new_script = document.createElement("script"),
      new_script.type = "text/javascript",
      new_script.async = true,
      new_script.id = "smcx-sdk",
      new_script.src =
        [
          "https:" === location.protocol ? "https://" : "http://",
          "widget.surveymonkey.com/collect/website/js/tRaiETqnLgj758hTBazgd9NxKf_2BhnTfDFrN34n_2BjT1Kk0sqrObugJL8ZXdb_2BaREa.js"
        ].join(""),
      last_script.parentNode.insertBefore(new_script, last_script)
    )
  }
)();

The fact that this adds a third-party script dependency to your website is problematic because it means that a security vulnerability in their infrastructure could lead to a complete compromise of your site, thanks to third-party scripts having full control over your website. Security issues aside though, this could also enable this third-party to violate your users' privacy expectations and extract any information displayed on your site for marketing purposes.

However, if you embed the snippet on a test page and inspect it with the developer tools, you will find that it actually creates an iframe:

<iframe
    width="500"
    height="500"
    frameborder="0"
    allowtransparency="true"
    src="https://www.surveymonkey.com/r/D3KDY6R?embedded=1"
></iframe>

and you can use that directly on your site without having to load their script.

Mixed content anti-pattern

As an aside, the script snippet they propose makes use of a common front-end anti-pattern:

"https:"===location.protocol?"https://":"http://"

This is presumably meant to avoid inserting an HTTP script element into an HTTPS page, since that would be considered mixed content and get blocked by browsers, however this is entirely unnecessary. One should only ever use the HTTPS version of such scripts anyways since an HTTP page never prohibits embedding HTTPS content.

In other words, the above code snippet can be simplified to:

"https://"

Restricting iframes

Thanks to defenses which have been added to the web platform recently, there are a few things that can be done to constrain iframes.

Firstly, you can choose to hide your full page URL from SurveyMonkey using the referrer policy:

referrerpolicy="strict-origin"

This mean seem harmless, but page URLs sometimes include sensitive information in the URL path or query string, for example, search terms that a user might have typed. The strict-origin policy will limit the referrer to your site's hostname, port and protocol.

Secondly, you can prevent the iframe from being able to access anything about its embedding page or to trigger popups and unwanted downloads using the sandbox attribute:

sandbox="allow-scripts allow-forms"

Ideally, the contents of this attribute would be empty so that all restrictions would be active, but SurveyMonkey is a JavaScript application and it of course needs to submit a form since that's the purpose of the widget.

Finally, a new experimental capability is making its way into browsers: feature policy. In the context of untrusted iframes, it enables developers to explicitly disable certain powerful features:

allow="accelerometer 'none';
       ambient-light-sensor 'none';
       camera 'none';
       display-capture 'none';
       document-domain 'none';
       fullscreen 'none';
       geolocation 'none';
       gyroscope 'none';
       magnetometer 'none';
       microphone 'none';
       midi 'none';
       payment 'none';
       usb 'none';
       vibrate 'none';
       vr 'none';
       webauthn 'none'"

Putting it all together, we end up with the following HTML snippet:

<iframe
    width="500"
    height="500"
    frameborder="0"
    allowtransparency="true"
    allow="accelerometer 'none'; ambient-light-sensor 'none';
           camera 'none'; display-capture 'none';
           document-domain 'none'; fullscreen 'none';
           geolocation 'none'; gyroscope 'none'; magnetometer 'none';
           microphone 'none'; midi 'none'; payment 'none'; usb 'none';
           vibrate 'none'; vr 'none'; webauthn 'none'"
    sandbox="allow-scripts allow-forms"
    referrerpolicy="strict-origin"
    src="https://www.surveymonkey.com/r/D3KDY6R?embedded=1"
></iframe>

Content Security Policy

Another advantage of using the iframe directly is that instead of loosening your site's Content Security Policy by adding all of the following:

script-src https://www.surveymonkey.com
img-src https://www.surveymonkey.com
frame-src https://www.surveymonkey.com

you can limit the extra directives to just the frame controls:

frame-src https://www.surveymonkey.com

CSP Embedded Enforcement would be another nice mechanism to make use of, but looking at SurveyMonkey's CSP policy:

Content-Security-Policy:
  default-src https: data: blob: 'unsafe-eval' 'unsafe-inline'
      wss://*.hotjar.com 'self';
  img-src https: http: data: blob: 'self';
  script-src https: 'unsafe-eval' 'unsafe-inline' http://www.google-analytics.com http://ajax.googleapis.com
      http://bat.bing.com http://static.hotjar.com http://www.googleadservices.com
      'self';
  style-src https: 'unsafe-inline' http://secure.surveymonkey.com 'self';
  report-uri https://csp.surveymonkey.com/report?e=true&c=prod&a=responseweb

it allows the injection of arbitrary Flash files, inline scripts, evals and any other scripts hosted on an HTTPS URL, which means that it doesn't really provide any meaningful security benefits.

Embedded enforcement is thefore not a usable security control in this particular example until SurveyMonkey gets a stricter CSP policy.

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