extend is a port of the classic extend() method from jQuery.
Affected versions of this package are vulnerable to Prototype Pollution. Utilities function can be tricked into modifying the prototype of "Object" when the attacker control part of the structure passed to these function. This can let an attacker add or modify existing property that will exist on all object.
Remediation
Upgrade extend to versions 2.0.2, 3.0.2 or higher.