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

Cross Site Scripting in CSS Stylesheets: Possibilities and Techniques

Cross-site scripting (XSS) is a web security vulnerability that allows attackers to inject malicious scripts into a web page, potentially compromising user data and account access. While XSS commonly targets JavaScript execution, it can also be exploited through CSS stylesheets.

Using CSS Stylesheets for XSS

In specific CSS implementations, it is possible to include JavaScript code within stylesheet files. Three primary methods have been identified:

1. expression(...) Directive: Enables the evaluation of JavaScript statements and incorporation of their results into CSS parameters.
2. url('javascript:...') Directive: Allows the injection of JavaScript into properties that support URL values.
3. Browser-Specific Features: Mozilla Firefox, for example, supports the -moz-binding mechanism, which enables invoking JavaScript through CSS.

Real-World Examples

A notable example of XSS via CSS stylesheets can be found in Firefox's use of XBL (Extensible Binding Language). It permitted the injection of JavaScript via CSS from files within the same domain.

Another technique described by ScaryBeastSecurity exploits the CSS parser to steal content from different domains, leveraging a vulnerability in how CSS handles cross-domain requests.

Protecting Against CSS XSS

To mitigate the risk of XSS via CSS stylesheets, several measures can be implemented:

• Enforce strict Content Security Policy (CSP) rules to prevent the loading of external scripts or stylesheets.
• Sanitize CSS inputs and remove malicious code.
• Disable browser-specific features that enable JavaScript execution through CSS, such as -moz-binding.
• Implement a web application firewall (WAF) to detect and block malicious CSS requests.

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