WhatsApp just took a hard new line against the malware industry, suing notorious Israeli surveillance contractor NSO Group for attacks on more than a thousand of its users. The case could mark a turning point in Silicon Valley's fight against private-sector espionage mercenaries. But before it can convince a court that NSO engaged in criminal hacking, WhatsApp may have to win a thorny legal argument—one that legal experts say could require some creative contortions.