Can CSS Stylesheets Enable Cross-Site Scripting Attacks?

Can CSS Stylesheets Host Cross-Site Scripting Exploits?

Question:

Can cross-site scripting (XSS) vulnerabilities be exploited through CSS stylesheets? If so, how can it be achieved using a malicious reference stylesheet, as opposed to inline style tags?

Answer:

According to the Browser Security Handbook, CSS implementations do indeed permit JavaScript code execution within stylesheets through three main methods:

1. Expression(...) directive: Evaluates JavaScript statements and utilizes their return values as CSS parameters.
2. Url('javascript:...') directive: Inserts JavaScript code into properties that support this directive.
3. Browser-specific features: Firefox's -moz-binding mechanism, for instance, allows JavaScript invocation in CSS.

In addition, StackOverflow users have noted that JavaScript can be injected into a page in Firefox via CSS using XBL (Extensible Binding Language). However, it's worth mentioning that the XBL file must originate from the same domain to prevent exploitation.

Another notable technique is described in the Scary Beast Security blog. By manipulating the CSS parser, it becomes possible to retrieve content from a different domain, although this differs slightly from the concept of cross-site scripting.

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