"Hess acknowledged that the bureau uses zero-days—the first time an official has done so. She said the trade-off is one the bureau wrestles with. "What is the greater good—to be able to identify a person who is threatening public safety?" Or to alert software makers to bugs that, if unpatched, could leave consumers vulnerable?
"How do we balance that?" she said. "That is a constant challenge for us.""
http://arstechnica.co.uk/tech-policy/2015/12/fbi-admits-it-uses-stingrays-zero-day-exploits/