defaults-deep node module before 0.2.4 suffers from a Modification of Assumed-Immutable Data (MAID) vulnerability, which allows a malicious user to modify the prototype of "Object" via __proto__, causing the addition or modification of an existing property that will exist on all objects.

A prototype pollution vulnerability was found in defaults-deep <=0.2.4 that would allow a malicious user to inject properties onto Object.prototype.