The NSA helps the companies find weaknesses in their products. But it also pays the companies not to fix some of them. Those weak spots give the agency an entry point for spying or attacking foreign governments that install the products in their intelligence agencies, their militaries, and their critical infrastructure. Microsoft, for instance, shares zero day vulnerabilities in its products with the NSA before releasing a public alert or a software patch, according to the company and U.S. officials. Cisco, one of the world’s top network equipment makers, leaves backdoors in its routers so they can be monitored by U.S. agencies, according to a cyber security professional who trains NSA employees in defensive techniques. And McAfee, the Internet security company, provides the NSA, the CIA, and the FBI with network traffic flows, analysis of malware, and information about hacking trends.