It sabotage

Earlier, the former British foreign secretary, Sir Malcolm Rifkind, said cyber attacks had worked in the past but he could not say if it sabotage had been behind Sunday’s failure. “There is a very strong belief that the US – through cyber methods – has been successful on several occasions in interrupting these sorts of tests and making them fail,” he said in an interview on BBC radio. (The GUardian.)

What is the antecedent of pronoun "it" in the text above?