Improper Validation of Specified Quantity in Input

Summary

fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 prior to 4.5.5 and 5.x prior to 5.5.7, the "DocTypeReader" in fast-xml-parser uses JavaScript truthy checks to evaluate "maxEntityCount" and "maxEntitySize" configuration limits. When a developer explicitly sets either limit to 0 intending to disallow all entities or restrict entity size to zero bytes the falsy nature of 0 in JavaScript causes the guard conditions to short-circuit, completely bypassing the limits. An attacker who can supply XML input to such an application can trigger unbounded entity expansion, leading to memory exhaustion and denial of service. This issue has been patched in versions 4.5.5 and 5.5.7.