CssAllowsArbitraryCodeExecution

(legacy summary: CSS allows binding of properties to arbitrary javascript expressions) (legacy labels: Attack-Vector)

CSS allows execution of unsanitized javascript?

Effect

Crafted CSS stylesheets can execute unsanitized javascript in the global scope on some browsers.

Background

CSS includes several mechanisms for changing the surrounding markup and executing expressions.

IE has an extension that allows execution of arbitrary javascript. The expression property is described at http://msdn2.microsoft.com/en-us/library/ms537634.aspx

Using the power of dynamic properties, it is now possible to declare property values not only as constants, but also as formulas. ... For scripting, a dynamic property can be any legal JScript or Microsoft Visual Basic Scripting Edition (VBScript) statement.

http://msdn2.microsoft.com/en-us/library/ms533503.aspx

binding allows binding to externally specified scripts

http://developer.mozilla.org/en/docs/CSS:-moz-binding & http://developer.mozilla.org/en/docs/XBL:XBL_1.0_Reference:Elements#binding

-moz-binding allows binding via the XML interface (also using data: URLs)

The content property allows specifying text that is included in the DOM but it is unknown whether this is exploitable if controllable by an attacker.

Assumptions

Untrusted code can generate style elements or style attributes or otherwise add arbitrary CSS rules and create DOM elements that trigger those rules.

Versions

IE 5 and later (but not IE 8 or later in "standards mode").

Mozilla/Firefox, versions not known.

Example

<div id='oDiv' style='left:expression(alert("hello"), 0)'>
Example DIV
</div>

node.style.cssText = 'left:expression(alert("hello"), 0)';

<input style='-moz-binding: url("http://www.mozilla.org/xbl/htmlBindings.xml#checkbox");'>

div {
  -moz-binding: url(data:text/xml;charset=utf-8,%3C%3Fxml%20version%3D%221.0%22%3F%3E%0A%3Cbindings%20id%3D%22xbltestBindings%22%20xmlns%3D%22http%3A//www.mozilla.org/xbl%22%3E%0A%20%20%3Cbinding%20id%3D%22xbltest%22%3E%3Ccontent%3EPASS%3C/content%3E%3C/binding%3E%0A%3C/bindings%3E%0A);
}

node.style.MozBinding = 'url("http://www.mozilla.org/xbl/htmlBindings.xml#checkbox")';

<ul>
  <li style="behavior:url(a1.htc) url(a2.htc)">List Item</li>
</ul>