CssImportsAllowUnsanitizedCodeExecution

(legacy summary: @import(<url>) can allow execution of script in unsanitized CSS) (legacy labels: Attack-Vector)

Reported by Michal Zalewski

@import can import unsanitized CSS

Effect

Allows import of arbitrary CSS which can execute arbitrary javascript.

Background

In a CSS file <style> tag, an @import declaration can be used to import styles from an external source specified by a URL relative to the URL of the CSS or HTML file containing the @import.

The data: protocol can be used to specify inline content in some browsers.

Assumptions

@import is allowed in style tags or third party CSS files and is not restricted to known safe CSS.

Versions

All

Example

Expression is IE specific, but the same could be applied to -moz-binding in Firefox. IE 7 does not support data: urls.

<style>
@import "data:text/css;charset=UTF-8,p%7Bwidth%3Aexpression%28alert%281%29%29%7D";
</style>

An @import with a HTTP url will also allow importing of unsanitized CSS.

IE has an undocumented extension that serves as a shorthand for @import:

    <style><!--@\ "http://www.finjan.com/mcrc/file.css";--></style>