When an HTTPS website references insecure (HTTP) resources, this is called mixed content.
Browsers prevent an HTTPS website from loading most insecure resources, like fonts, scripts, etc. Migrating an existing website from HTTP to HTTPS means identifying and fixing or replacing mixed content.
Mixed content comes in two varieties:
Passive mixed content includes resources whose impact on the page’s overall behavior is more minimal, such as images, audio, and video. Browsers will load passive mixed content, but will typically change the HTTPS indicator.
In Chrome, a website indicator for passive mixed content looks like this:
Every website’s mixed content situation will be different, but the general approach is:
https://for your website, but don’t force a redirect. Continue to present the
http://version as the canonical URL to search engines.
- Identify the most obvious and widespread pieces of mixed content by loading your website in a browser over
https://and observing breakages. Chrome, Opera, and Firefox will log any mixed content warnings to the console, which should point out necessary site-wide changes. Use these to secure your resource links.
- After fixing them, tackle the long tail by scanning your code and crawling your website.
- Finally, force the redirect to HTTPS, turn on HSTS, and tell search engines that your new URL starts with
Note: the below instructions use tools optimized for an OS X or Linux environment. Documentation for Windows-based tools would be a welcome contribution to this guide.
Linking to resources securely
Most commonly used third party services, such as Google Analytics or AddThis, will automatically adapt when migrating to HTTPS.
Other services may require manual updates, but have an
https:// version ready:
<link href="https://fonts.googleapis.com/css?family=Open+Sans" rel="stylesheet">
Generally speaking, for content on your own domain, stick to site-relative URLs wherever possible:
<img src="/media/my-picture.png" />
When migrating a site with a lot of user- or staff-submitted content (e.g. a blog), you may find media hotlinked from a third-party domain which doesn’t support HTTPS.
This is a great opportunity to improve your website’s privacy and lessen your dependency on third parties, by copying those media files to your own server instead and hosting them yourself.
Scanning your code
After identifying and fixing the obvious issues, you can scan your website’s files for leads. On a Mac or Linux-based system,
grep is very handy:
Images and scripts:
grep -r "src=\"http:" *
Stylesheets and fonts:
grep -r "href=\"http:" * | grep "<link"
CSS imports and references:
grep -r "url(\"http:"
http: references and try to exclude hyperlinks in HTML or Markdown:
grep -r "http:" | grep -v "href=\"http:" grep -r "http:" | grep -v "](http:"
Crawling your website
mixed-content-scan is a very handy command line tool that can crawl an
https:// website to see if it contains any references to insecure resources. This is especially helpful if your content is primarily managed in a CMS.
You should see something like this:
[2015-03-15 16:56:48] MCS.NOTICE: Scanning https://https.cio.gov/   [2015-03-15 16:56:49] MCS.INFO: 00000 - https://https.cio.gov/   [2015-03-15 16:56:49] MCS.INFO: 00001 - https://https.cio.gov/faq/   [2015-03-15 16:56:49] MCS.INFO: 00002 - https://https.cio.gov/hsts/   [2015-03-15 16:56:49] MCS.INFO: 00003 - https://https.cio.gov/resources/   [2015-03-15 16:56:49] MCS.NOTICE: Scanned 4 pages for Mixed Content  
Any discovered mixed content will be listed as a
WARNING. You can also get the results as newline-separated JSON objects:
mixed-content-scan https://https.cio.gov --format=json
Automatically detecting mixed content
Site owners also have the option of using a Content Security Policy header that will instruct browsers to ping a given URL with information about any observed mixed content warnings.
A CSP policy that only specified pinging for mixed content warnings might look like:
Content-Security-Policy-Report-Only: default-src https:; report-uri https://example.com/reporting/endpoint
CSP reporting, especially for larger services, is an advanced approach that will likely require planning and tuning by a dedicated development team.
- An engineer describes Twitter’s approach as of 2014.
report-uri.io, which offers a hosted reporting URI for CSP violations (or public key pinning violations) and accompanying dashboard.
- A newly developed CSP extension, Upgrade Insecure Requests, will instruct browsers to automatically upgrade referenced HTTP URLs to HTTPS URLs without triggering mixed content detection. This extension is not finalized, and as of June 2015 is only available in Chrome.
Why do browsers block mixed content?
If mixed content were not blocked, an attacker could control the main website by conducting a MITM attack against any of its active resources.
Even with passive content like images, attackers can manipulate what the page looks like, and so the yellow-lock icon is intended to communicate that security has been weakened and user confidence should be reduced. In addition, an attacker will be able to read any cookies for that domain which do have the
Secure flag, and set cookies.
When a website is accessible over
http://, loading other insecure resources does not generate any sort of warning, and so websites operating over plain HTTP often accumulate many of these sub-resources.
Security considerations for third party content
Incorporating or loading content from third party domains creates an additional attack vector.
Even if a page has all page elements loaded over HTTPS, variations in HTTPS configurations could result in security vulnerabilities. For example, if ‘foo.gov’ loads a page element over HTTPS from ‘bar.com’ but ‘bar.com’ is not as fastidious with its HTTPS/TLS configuration, the page element from ‘bar.com’ may allow injection of malicious software into the page.
For example, if ‘bar.com’ uses a TLS configuration that is known to be weak, a malicious network adversary may be able to modify or replace the page element to inject software that could read the page contents or, potentially, exploit browser vulnerabilities and accomplish more global access to the client device. Accordingly, just as it’s important to regularly evaluate the HTTPS/TLS configuration of US government websites, it will be important to also evaluate the configurations of the domains that serve third-party page elements.
Note that this is still a strict improvement over incorporating content third party domains over unencrypted HTTP. Attacks on the privacy, integrity, and security of connections to third party domains over unencrypted HTTP are trivial.