Every serializable class that has private mutable instance variables must defensively copy them in the readObject() method. An adversary attacker can tamper with the serialized form of such a class, appending extra references to the byte stream. When deserialized, this byte stream could allow the creation of a class instance whose internal variable references are controlled by the attacker. Consequently, this allows the instance of the container class to mutate and violate its class invariants.
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