When a user visits a page served over HTTPS, their connection with the web server is encrypted with TLS and is therefore safeguarded from most sniffers and man-in-the-middle attacks. An HTTPS page that includes content fetched using cleartext HTTP is called a mixed content page. Pages like this are only partially encrypted, leaving the unencrypted content accessible to sniffers and man-in-the-middle attackers. That leaves the pages unsafe.
There are two categories for mixed content: mixed passive/display content and mixed active content. The difference lies in the threat level of the worst case scenario if content is rewritten as part of a man-in-the-middle attack. In the case of passive content, the threat is lower (the page may contain misleading content, or the user's cookies may be stolen). In the case of active content, the threat can lead to phishing, sensitive data disclosure, redirection to malicious sites, etc.
Mixed passive/display content is content served over HTTP that is included in an HTTPS webpage, but that cannot alter other portions of the webpage. For example, an attacker could replace an image served over HTTP with an inappropriate image or message to the user. The attacker could also infer information about the user's activities by watching which images are served to the user; often images are only served on a specific page within a website. If the attacker observes HTTP requests to certain images, they could determine which webpage the user is visiting.
Passive content list
This section lists all types of HTTP requests which are considered passive content:
Mixed active content is content that has access to all or parts of the Document Object Model of the HTTPS page. This type of mixed content can alter the behavior of the HTTPS page and potentially steal sensitive data from the user. Hence, in addition to the risks described for mixed display content above, mixed active content is vulnerable to a few other attack vectors.
The risk involved with mixed content does depend on the type of website the user is visiting and how sensitive the data exposed to that site may be. The webpage may have public data visible to the world or private data visible only when authenticated. If the webpage is public and has no sensitive data about the user, using mixed active content still provides the attacker with the opportunity to redirect the user to other HTTP pages and steal HTTP cookies from those sites.
Active content examples
This section lists some types of HTTP requests which are considered active content:
hrefattribute) (this includes CSS stylesheets)
- All cases in CSS where a
<url>value is used (
background-image, and so forth).
Other resource types like web fonts and workers may be considered active mixed content, as they are in Chrome.
The Firefox Web Console displays a mixed content warning message in the Net pane when a page on your website has this issue. The mixed content resource that was loaded via HTTP will show up in red, along with the text "mixed content", which links to this page.
As well as finding these warnings in the Web Console, you could use Content Security Policy (CSP) to report issues. You could also use an online crawler like SSL-check or Missing Padlock that will check your website recursively and find links to insecure content.
Starting in Firefox 23, mixed active content is blocked by default (and mixed display content can be blocked by setting a preference). To make it easier for web developers to find mixed content errors, all blocked mixed content requests are logged to the Security pane of the Web Console, as seen below:
Note: Since Firefox 55, the loading of mixed content is allowed on http://127.0.0.1/ (see bug 903966). Chrome allows mixed content on http://127.0.0.1/ and http://localhost/. Safari does not allow any mixed content.