The devalue library versions 4.0.0 up to but not including 5.6.4 have a behavior where the functions devalue.parse and devalue.unflatten can produce objects containing own properties named __proto__. While this behavior alone is not a security vulnerability, it can lead to prototype pollution if downstream code improperly handles these objects, such as by using Object.assign. This can result in the target object's prototype being modified unintentionally. No known exploits are reported in the wild, and the severity is considered low.