HTML Tags in Javascript Strings can allow Unsanitized Script Execution

Effect

Unsanitized code can be executed in the global scope

Background

Javascript can be embedded in HTML, or served in separate files.

In HTML, there is one way

<script>
// Script proceeds until the string "</script" is seen.
// Strictly it is allowed to end when "</" is seen, but all browsers support the looser
// behavior.
</script>

If the comment markers {<!-- ... -->} are seen around the contents of a script tag, they are ignored. HTML 4 does not actually treat the content of the comment as a comment since the SCRIPT tag's content is specified as CDATA.

In XHTML, a script tag is not always CDATA like HTML, but may be composed from a series of Text nodes and CDATA sections

<script>
1 &lt; 2;  // Have to escape HTML special characters
<![CDATA[[
3 < 4;  // Now I don't
]]>
5 &lt; 6;  // Now I do again
</script>

http://www.w3.org/TR/1998/REC-xml-19980210#sec-entity-decl describes external entity definitions. External entities can be used in XHTML to perform javascript imports:

<!DOCTYPE ...
  <!ENTITY some-source SYSTEM "my-script.js">
>
<script>
&my-script;
</script>

Assumptions

Rewritten javascript allows any of the operators <, <<, <<<, &, or && to be followed immediately by an identifier, a slash (/), an open square bracket ([), or a bang !.

OR

The characters < and & in string literals are output without escaping, and string literals are not split into a concatenation after these characters.

Versions

All

Example

'</script><script>alert("hello world");//'

when embedded in an HTML page naively.

"]]]>&quot;; alert('hello world');//<script>//"

when embedded in an XHTML page as

<script><![CDATA["]]]>&quot;; alert('hello world');//<script>//"]]></script>

var script;
1</script />2;
// Can now insert arbitrary html into the document
var iframe, src;
1<script ;
src="foo" > "bar";

when embedded in normal HTML. Note, no tag beginning or end appear inside string literals.

Escaping script enables any number of external entities as described above.