billion laughs en · NOUN
Etymology
In the most frequently cited example of such an attack, the first entity is the string lol (“laugh out loud”).
Meanings
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(plural, plural-only, qualifier:computer security) A type of denial-of-service attack which is aimed at parsers of XML documents, based on defining entities that expand to large numbers of copies of other entities.
The XML billion laughs DoS attack simply starts by declaring an XML document with an entity named lol (hence the name laugh gets associated with it, but in a general case it can be any valid name).
2016, Prakhar Prasad, Mastering Modern Web Penetration Testing, Packt Publishing Ltd, →ISBN, page 193: