The writeObject method of java.math.BigInteger is not threadsafe. It mutates and accesses the instance field "magnitude" without any synchronization. If an instance is being serialized by two threads concurrently, the serialized form could be corrupted: for example, "magnitude" might be set to null by the first thread in between the point that the second thread has assigned the result of magSerializedForm() to the field and the point that the execution of defaultWriteObject() reads the field for serializing, in which case the value serialized for the field will be null instead of the byte[]-encoded magnitude of the number.