Mixed Content: Why Browsers Block It

TL;DR

Browsers block ‘mixed content’ – loading secure (HTTPS) pages with insecure (HTTP) resources like images, scripts or stylesheets – to protect your privacy and security. It’s a good thing! This guide explains why and how to fix it.

Why Browsers Block Mixed Content

1. Security Risks: Imagine you visit a bank website (HTTPS). If an image loads over HTTP, a hacker could intercept that image request and redirect you to a fake login page. They can steal your username and password because the browser trusts the secure site but doesn’t flag the insecure resource as dangerous enough on its own.
2. Data Tampering: An attacker could modify the HTTP content (e.g., change an image) before it reaches your browser, potentially altering what you see or injecting malicious code.
3. Privacy Concerns: Even seemingly harmless resources can reveal information about your browsing habits to attackers monitoring the insecure connection.
4. Browser Enforcement: Modern browsers are increasingly strict about mixed content. They’re doing this to improve overall web security for everyone.

How Browsers Handle Mixed Content

• Blocking: The most common action. The browser prevents the insecure resource from loading, often displaying a broken image icon or an error message.
• Warnings: Some browsers might show a warning in the address bar indicating that mixed content is present.
• Passive vs Active Content:
  • Passive Content (Images, Audio, Video): Usually blocked but often less critical. Browsers *used* to allow passive content by default, but are now moving towards blocking it too for increased security.
  • Active Content (Scripts, Stylesheets, Iframes): Almost always blocked because they can directly affect the page’s behaviour and pose a greater risk.

Fixing Mixed Content Issues

1. Update HTTP Resources to HTTPS: This is the *best* solution.
   • If you control the server hosting the insecure resources, enable HTTPS. Let’s Encrypt (https://letsencrypt.org/) provides free SSL/TLS certificates.
   • Contact the provider of the resource if it’s hosted elsewhere and ask them to support HTTPS.
2. Replace HTTP Links with HTTPS: In your website’s code, change all instances of http:// to https://.

   <img src="http://example.com/image.jpg" alt="Example Image">

   becomes

   <img src="https://example.com/image.jpg" alt="Example Image">

3. Use Relative URLs: If the resource is on the same domain, use relative paths.

   <img src="/images/image.jpg" alt="Example Image">

4. Content Security Policy (CSP): A more advanced technique to control which resources your browser is allowed to load. You can configure CSP headers on your server.

   Example:

   Content-Security-Policy: upgrade-insecure-requests

   This tells the browser to automatically upgrade all HTTP requests to HTTPS where possible.

5. Browser Developer Tools: Use your browser’s developer tools (usually accessed by pressing F12) to identify mixed content errors. The ‘Console’ tab will often show warnings about blocked resources.
   • Chrome/Edge: Look for messages like “Mixed Content: The page at [URL] was loaded over HTTPS, but requested an insecure resource [URL].”
   • Firefox: Similar error messages in the console.

Checking Your Site

Use online tools to scan your website for mixed content issues:

• Why No Padlock?: https://www.whynopadlock.com/
• SSL Server Test: https://www.sslservertest.com/ (includes mixed content checks)