The CIA has admitted that too many informants are being killed, captured or turned in a top secret memo to their spies across the world. The unusual cable, sent to all CIA stations and bases, said the counterintelligence mission center had analyzed dozens of cases over the last several years. The memo gave an exact number of informants killed, classified information not usually shared in such cables, according to the New York Times . Former officials have also disclosed that China and Iran cracked the agency's classified communications system, or 'covcom', and executed informants in those networks while others had…