andermoran/Objective-C-code-injection-example-in-macOS-Sierra

Objective-C code injection example in macOS Sierra

What is this?

I have tinkered with code injection on iOS and decided to dabble with code injection on macOS. I made an Objective-C class and used DYLD_INSERT_LIBRARIES for injection.

What's going on?

This very very basic example of code injection swizzles -(NSString *)getName. This is a method of the SampleUser class. When the executable is run normally, the output will be

name = John Doe

age = 20

When the executable is run with the dynamic library injected into it, the output be

name = Alan Turing

age = 20

Look inside the SampleUserOverrides.m file to see what the new method does :)

How do I run this?

To compile the executable

gcc -fobjc-arc -framework Foundation -o SampleUserTester SampleUserTester.m SampleUser.m

To create the dynamic library for injection

gcc -framework Foundation -o SampleUserOverrides.dylib -dynamiclib SampleUserOverrides.m

To run the executable and inject the dynamic library

DYLD_INSERT_LIBRARIES=/PATH/TO/SampleUserOverrides.dylib ./SampleUserTester

Need more info?

Check out Timac's article for a more in depth explanation of what is going on.