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Comment by K0nserv | Hacker News Reader

AFAIK the combination of NSSelectorFromString and NSClassFromString would allow you to build and make calls from obfuscated strings during runtime. Since Objective-C uses message passing this is non trivial to catch. Compared to C were you must explicitly reference symbols that are easy to automatically check for detecting the use of third party APIs is more difficult.

I am not sure if you have to explicitly link against certain frameworks/dylibs though, someone with more knowledge feel free to correct me

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The article touches on how to dynamically link into a framework/dylib at runtime with dlopen(3) and dlsym(3) to resolve it's symbols address space.

I've always wondered why Apple doesn't run all apps against a "debug" build of iOS that asserts that the caller of private APIs is itself internal/private to Apple, but instead relies on something akin to grepping the output of strings(1)

This seems like a good approach, but you can still get around it by detecting apple reviews and not doing anything with private APIs during them

It's quite trivial to setup a method that implements "NSSelectorFromString" [1] etc.. and have it read from a Json payload send from a server. It's very hard for Apple to check for that unless they have active monitoring on apps after the review process.

[1] https://developer.apple.com/library/ios/documentation/Genera...

Right. Any runtime behavior can be altered by observed state from outside the phone.

There's even a paper on intentionally inserting security flaws into your code and then exploiting them from your own server to change execution patterns:

https://www.usenix.org/conference/usenixsecurity13/technical...

Ultimately, you need to enforce access control instead of just trying to detect problems a priori. Apple's sandbox is a great start to that, and I expect they'll keep improving it to block apps like these.

The state doesn't even have to be from outside the phone. You could have an internal timer that kicks off 2 weeks after you've submitted to the App Store (to allow for unexpected delays) and switches on the evil behavior.

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