assign-deep node module before 0.4.7 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.

assign-deep is vulnerable to Prototype Pollution in versions before 0.4.8 and version 1.0.0. The function assign-deep could be tricked into adding or modifying properties of Object.prototype using either a constructor or a _proto_ payload.