TL;DR
Yes, there are vulnerabilities when making cross-domain HTTP requests even during an SSL connection (HTTPS). These risks mainly stem from mixed content issues and potential for Man-in-the-Middle attacks if not handled correctly. This guide explains the problems and how to fix them.
Understanding the Problem
When your website uses HTTPS, but loads resources (images, scripts, stylesheets) over HTTP from a different domain, you create security weaknesses. Even with SSL in place for *your* site, those unencrypted requests can be intercepted and modified.
Step-by-Step Solution
- Identify Mixed Content: The first step is to find any resources loaded over HTTP on your HTTPS pages. Your browser’s developer tools are key here.
- Chrome DevTools: Open Developer Tools (F12), go to the ‘Console’ tab. Look for warnings about “Mixed Content”.
- Firefox DevTools: Similar process – open Developer Tools, check the ‘Console’ tab for mixed content errors.
- Fix Mixed Content – Update URLs: The simplest solution is to change all HTTP links to HTTPS.
- Example: Change
to
- If the resource *doesn’t* support HTTPS, see Step 4.
- Example: Change
- Content Security Policy (CSP): CSP helps control which resources your browser is allowed to load.
- Add a CSP header to your web server configuration. This tells the browser what domains are trusted for different resource types. A basic example:
Content-Security-Policy: default-src https: - More specific policies can be defined (e.g., allowing images only from a certain domain).
- Add a CSP header to your web server configuration. This tells the browser what domains are trusted for different resource types. A basic example:
- Dealing with Resources That Don’t Support HTTPS: If a third-party resource *absolutely* cannot be served over HTTPS:
- Subdomain Isolation: Serve the insecure content from a separate subdomain (e.g., http://insecure.example.com). This limits the impact on your main domain.
- Avoid if Possible: Seriously consider finding an alternative resource that *does* support HTTPS. This is the best solution.
- HSTS (HTTP Strict Transport Security): HSTS forces browsers to only connect to your site over HTTPS, preventing downgrade attacks.
- Configure your web server to send the HSTS header. Example:
Strict-Transport-Security: max-age=31536000; includeSubDomains - Be careful with initial configuration – ensure all subdomains support HTTPS before enabling
includeSubDomains.
- Configure your web server to send the HSTS header. Example:
- Regular Security Scanning: Use automated tools to regularly scan your website for mixed content and other vulnerabilities.
- Online Scanners: Tools like SSL Labs’ SSL Server Test (https://www.ssllabs.com/ssltest/) can identify issues.
- Browser Extensions: Several browser extensions help detect mixed content in real-time.
Man-in-the-Middle (MitM) Risks
Even with SSL, a MitM attack is possible if the user’s computer has an invalid or compromised certificate authority. Mixed content makes this easier as the HTTP requests are not verified.