Introduction
After enabling HTTPS on a website, many teams are surprised to see browser warnings saying “Mixed Content” or notices that the site is “Not fully secure.” Users may see a lock icon with a warning, some features may stop working, and browsers may even block parts of the page. This often happens right after SSL setup, migration, or deployment.
In simple words, mixed content means that your website is loaded over HTTPS, but some parts of the page are still loaded over HTTP. Browsers consider this unsafe because it breaks the security promise of HTTPS. This article explains what mixed content is, why it happens, and how to fix it properly using simple language and real-world examples.
What Mixed Content Actually Means
Mixed content occurs when a secure HTTPS page loads insecure resources using HTTP.
The main page is encrypted, but images, scripts, stylesheets, fonts, or API calls are not. This creates a security gap where attackers could intercept or modify those insecure resources.
For example, your website loads over https://example.com, but an image is loaded from http://example.com/image.jpg. The browser flags this as mixed content.
Why Browsers Block Mixed Content
Modern browsers are strict about security.
If a page is HTTPS, browsers expect all resources to be HTTPS as well. When insecure content is detected, browsers either warn users or block the content entirely.
For example, JavaScript files loaded over HTTP are usually blocked completely because they can be used to inject malicious code. Images may still load but trigger warnings.
This is why fixing mixed content is important for both security and user trust.
Common Causes of Mixed Content
Mixed content usually appears due to old configurations or partial migrations.
Hardcoded HTTP URLs inside HTML, CSS, or JavaScript files are the most common cause. Another common reason is third-party services that still use HTTP.
For example, a website migrated to HTTPS but still references old image URLs stored in the database or CMS content.
Images and Media Loaded Over HTTP
Images, videos, and media files are frequent sources of mixed content warnings.
Even though images seem harmless, loading them over HTTP breaks full encryption.
For example, blog posts may contain image URLs starting with http:// from older content. Browsers warn users even if the main page is secure.
Updating image URLs to HTTPS or using protocol-relative paths fixes this issue.
JavaScript and CSS Files Over HTTP
Scripts and stylesheets are considered active content and are more dangerous.
Browsers often block these resources completely when loaded over HTTP.
For example, a JavaScript analytics file loaded via HTTP will be blocked, causing features or tracking to stop working.
All script and CSS URLs must use HTTPS to avoid mixed content errors.
API Calls Made Over HTTP
Frontend applications often call backend APIs.
If the frontend runs on HTTPS but calls an HTTP API endpoint, browsers block the request.
For example, a secure frontend calling http://api.example.com/data will fail, even if the API works correctly.
The backend API must also be served over HTTPS.
Third-Party Services Without HTTPS
Some third-party services may still provide HTTP-only endpoints.
This commonly affects old analytics scripts, ads, fonts, or tracking pixels.
For example, an external font loaded from an HTTP URL triggers mixed content warnings.
Replacing these services with HTTPS-compatible alternatives solves the issue.
Incorrect CDN or Asset Configuration
CDNs may be configured incorrectly after HTTPS migration.
Assets may still be served over HTTP due to outdated CDN settings.
For example, a CDN origin URL is set to HTTP, causing assets to load insecurely even though the site is HTTPS.
Updating CDN settings to use HTTPS fixes this problem.
Redirects That Downgrade to HTTP
Sometimes resources are requested over HTTPS but redirected to HTTP.
This still counts as mixed content.
For example, a script URL starts with HTTPS but redirects to HTTP due to server misconfiguration.
Ensuring proper HTTPS redirects prevents this issue.
How to Find Mixed Content Issues
Browsers provide helpful tools to identify mixed content.
The developer console clearly lists blocked or insecure resources.
For example, opening the browser console shows which URLs are loaded over HTTP, making it easier to fix them one by one.
Fix Hardcoded HTTP URLs
Hardcoded HTTP URLs should be replaced with HTTPS.
This includes links inside templates, scripts, configuration files, and database content.
For example, updating all resource URLs to start with https:// ensures secure loading.
Use HTTPS Everywhere
All parts of the system must support HTTPS.
This includes frontend, backend APIs, CDNs, third-party services, and internal calls.
Consistency is key to avoiding mixed content.
Enable Automatic HTTPS Redirects Carefully
Redirecting HTTP to HTTPS helps, but it does not fix mixed content by itself.
If a resource is requested via HTTP, the browser may block it before the redirect happens.
The correct fix is to change the resource URL to HTTPS, not rely on redirects.
Clear Cache After Fixing Issues
Browsers and CDNs cache resources.
After fixing mixed content, cached insecure resources may still load.
Clearing browser cache or purging CDN cache ensures users see the updated secure version.
Test Across Browsers and Devices
Different browsers handle mixed content slightly differently.
Testing across browsers ensures the site is fully secure everywhere.
For example, a site may work in one browser but show warnings in another.
Summary
Mixed content warnings appear when an HTTPS website loads some resources over HTTP. Common causes include hardcoded URLs, insecure images or scripts, HTTP API calls, third-party services, CDN misconfiguration, and redirects to HTTP. Browsers block or warn about mixed content to protect users. By ensuring all resources use HTTPS, fixing hardcoded links, updating APIs and third-party services, and clearing caches, teams can fully secure their websites and restore user trust.