How Can Cross-Site Scripting (XSS) Be Exploited Through CSS Stylesheets?

Understanding Cross Site Scripting in CSS Stylesheets

Cross site scripting (XSS) is a malicious technique that allows attackers to inject malicious code into web pages, potentially compromising user data and system security. While XSS is often associated with JavaScript, it is possible to exploit vulnerabilities in CSS stylesheets as well.

How is XSS Possible in CSS Stylesheets?

CSS stylesheets are typically defined in external files referenced by web pages. This external linking mechanism can introduce vulnerabilities if the referenced stylesheet is compromised.

As outlined in the browser security handbook, there are several methods to execute malicious JavaScript within CSS stylesheets:

• Using the expression(...) directive to evaluate arbitrary JavaScript statements.
• Using the url('javascript:...') directive on properties that support it.
• Invoking browser-specific features such as the -moz-binding mechanism of Firefox.

Additionally, in Firefox, XBL (Extensible Binding Language) can be employed to inject JavaScript into a page via CSS. However, this method requires the XBL file to reside in the same domain (as noted in the StackOverflow thread mentioned by the answer).

Other Abuse of CSS

While not directly related to XSS, another technique is worth mentioning: misusing the CSS parser to steal content from different domains. This is described in the "Generic Cross-Browser Cross-Domain" article.

Protecting Against XSS in CSS

To mitigate XSS vulnerabilities in CSS, website developers should:

• Sanitize CSS files before referencing them in web pages.
• Ensure that trusted parties provide referenced stylesheets.
• Use browser-level security policies to restrict cross-site resource loading.

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