For reference I've googled every combination of "nsa apple mobile OS leak" I could think of and couldn't find a primary source.
http://www.spiegel.de/international/world/how-the-nsa-spies-...
Helps to type in just what you want and what will specifically have your answer. Mobile will give you garbage most of the time. Apple as well. A technical document will usually reference iOS. Also, you can use quotes to ensure something appears.
Interesting enough, me typing what you typed into Google still led to same leak and others showing potential backdoors. Hmmm.