There is no such thing as bulletproof security. If a well funded, technically competent and determined adversary is targeting you, they will get in. Your job is to make their life as difficult as possible by using passwords that are complex (difficult to guess) and by keeping your software up to date.
Apple has been a good steward of IOS security and regularly releases patches to protect its user base. Today we were gifted IOS 10.2.1 which is an out of plan upgrade I recommend you download asap.
IOS 10.2.1 includes some important security protection that you definitely want to get. These security fixes touch WebKit (the rendering engine for Safari) and protect against arbitrary code execution using kernel privilege (aka an exploit using this flaw could take complete control of your device).
This complete control thing is why you need to download it now. A skilled threat actor could use this to install/delete apps, copy files and spy on you. The Webkit flaws also allow an attacker to run arbitrary code.
Looks like many of the vulnerabilities were discovered by Google's Project Zero security research team. Obviously finding these required extremely skilled professionals but these high grade specialists work on both sides of the fence (some are white hats and others black hats). Due to the nature and complexity of the vulnerabilities, anyone exploiting them would be a nation state actor but an ounce of protection is worth a pound of cure.
To upgrade IOS, open the Settings applet and choose General > Software Update