We all know while using a new iOS app it might ask for information like location or access to the camera. Those are both features we can deny to the app. There are, however, features apps can access without permission. For example I already know apps can get information like the name of your phone "Mason's iPhone" or the storage available in the device. What are some features apps have access to that doesn't need to be granted permission?
This might not be exactly what you're looking for but...
Having your iPhone near your keyboard can make you vulnerable to keylogging. Before you call me crazy, I've actually conducted this experiment using neural networks and an Android's accelerometer and it could 80% guess what region of the keyboard the key press came from and could guess within a few keys away fairly often.
Privileges for reading data from the accelerometer are given by default to both iPhone and Android devices, and so a device making use of these readings could potentially figure out when you type in a URL, what passwords you type into the webpages you visit, and combining the probabilistic regions of the key presses with a dictionary attack or a common passwords list would easily break passwords in minutes to seconds, no issue.