40044—Exponential Entity Expansion (Billion Laughs Attack)

An exponential entity expansion, or “billion laughs” attack is a type of denial-of-service (DoS) attack. It is aimed at parsers of markup languages like XML or YAML that allow macro expansions.

Defenses against this kind of attack include capping the memory allocated in an individual parser if loss of the document is acceptable, or treating entities symbolically and expanding them lazily only when (and to the extent) their content is to be used.