Goldshlager highlights the similitude of the XML quadratic blowup attack with the Billion Laughs attack, it basically exploits the use of entity expansion, this means that it replicates one large entity using a couple thousand characters repeatedly.
“A medium-sized XML document of approximately two hundred kilobytes may require anywhere within the range of one hundred MB to several GB of memory. When the attack is combined with a particular level of nested expansion, an attacker is then able to achieve a higher ratio of success.”
In the following example provided by the expert, if the attacker defines the entity “&x;” as 55,000 characters long, and uses this entity 55,000 times inside the XML “DoS” element, the parser will expand to 2.5 GB the document causing the saturation of resources of targeted website.
<?xml version=”1.0″?> <!DOCTYPE DoS [!<ENTITY a "xxxxxxxxxxxxxxxxx...">]> <DoS>&x;&x;&x;&x;&x;&x;&x;&x;&x;…</DoS>
Following a video Proof of Concept of the attack on WordPress published by Goldshlager, while the PoC Exploit: (128MB Memory limit) is available at the address below