The centrifuges themselves are equipped with firmware chips, and in their original form those chips are (a) flash-upgradable and (b) not secured against an "unauthorised" flash upgrade. That makes them vulnerable to a worm that secretly re-writes the firmware with code that then instructs the centrifuge to spin itself to destruction.
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Erdogan: Turkey spent $10 billion on Syrian refugee crisis compared to UN's $455 million (Reuters)
from the article: Report: Israel tested Iran-bound Stuxnet worm in Dimona nuclear plant