Chinese state television has broadcast footage of what two experts on the Chinese military say appears to be a military institute demonstrating software designed to attack websites in the U.S. Although it could be a decade old or a mock-up, the 10-second segment—part of a longer report on cybersecurity—appears to be a rare example of an official source contradicting China'a repeated assertions that it doesn't engage in cyberattacks, according to Andrew Erickson and Gabe Collins of the China SignPost analytical service, which specializes in military matters. The slightest suggestion that the Chinese military has attacked U.S. websites is highly sensitive,...