Macos Objective C Analysis

macOS Objective-C Analysis

A skill for analyzing Objective-C code and binaries in macOS security research and reverse engineering.

When to Use This Skill

Use this skill when:

• Analyzing macOS/iOS binaries for security research
• Reading or understanding Objective-C source code
• Using class-dump to extract class information from Mach-O binaries
• Investigating app internals for privilege escalation or vulnerability research
• Working with Objective-C runtime concepts in security contexts
• Understanding class declarations, methods, and instance variables in compiled binaries

Core Concepts

Binary Analysis with class-dump

Objective-C programs retain their class declarations when compiled into Mach-O binaries. This includes:

• Class names
• Class methods
• Instance variables

Extract this information using:

class-dump <binary_path>
class-dump -H <binary_path>  # Include header output
class-dump -r <binary_path>  # Recursive for frameworks

Note: Class names may be obfuscated to complicate reverse engineering.

Class Structure

Interface Declaration

@interface MyVehicle : NSObject

@property NSString *vehicleType;
@property int numberOfWheels;

- (void)startEngine;
- (void)addWheels:(int)value;

@end

Implementation

@implementation MyVehicle : NSObject

- (void)startEngine {
    NSLog(@"Engine started");
}

- (void)addWheels:(int)value {
    self.numberOfWheels += value;
}

@end

Object Instantiation

// Allocate and initialize
MyVehicle *newVehicle = [[MyVehicle alloc] init];

// Shorthand
MyVehicle *newVehicle = [MyVehicle new];

// Call methods
[newVehicle addWheels:4];

Method Types

Property Access

// Dot notation (setter)
newVehicle.numberOfWheels = 2;

// Method call (setter)
[newVehicle setNumberOfWheels:3];

// Dot notation (getter)
NSLog(@"Wheels: %i", newVehicle.numberOfWheels);

// Method call (getter)
NSLog(@"Wheels: %i", [newVehicle numberOfWheels]);

Instance Variables

Access properties directly using underscore prefix:

- (void)makeLongTruck {
    _numberOfWheels = 10000;
    NSLog(@"Wheels: %i", self.numberOfWheels);
}

Protocols

Protocols define method contracts (no properties):

@protocol myVehicleProtocol
- (void)startEngine;           // mandatory (default)
@required
- (void)addWheels:(int)value;  // mandatory
@optional
- (void)makeLongTruck;         // optional
@end

@interface MyVehicle : NSObject <myVehicleProtocol>
@end

Common Classes for Analysis

NSString

// String literals
NSString *title = @"The Catcher in the Rye";

// From C string
NSString *author = [NSString stringWithCString:"J.D. Salinger" encoding:NSUTF8StringEncoding];

// Format strings
NSString *desc = [NSString stringWithFormat:@"%@ by %@", title, author];

// Mutable strings
NSMutableString *mutable = [NSMutableString stringWithString:@"The book "];
[mutable appendString:title];

NSNumber

// Character
NSNumber *letter = @'Z';

// Integers
NSNumber *num = @42;
NSNumber *numLong = @42L;

// Floats
NSNumber *pi = @3.14159;

// Booleans
NSNumber *yes = @YES;
NSNumber *no = @NO;

Collections

// Arrays
NSArray *immutable = @[@"red", @"green", @"blue"];
NSMutableArray *mutable = [NSMutableArray array];
[mutable addObject:@"red"];

// Sets
NSSet *immutableSet = [NSSet setWithArray:@[@"apple", @"banana"]];
NSMutableSet *mutableSet = [NSMutableSet set];

// Dictionaries
NSDictionary *dict = @{
    @"apple": @"red",
    @"banana": @"yellow"
};
NSMutableDictionary *mutDict = [NSMutableDictionary dictionaryWithDictionary:dict];

Blocks (Closures)

// Define block
int (^suma)(int, int) = ^(int a, int b) {
    return a + b;
};
NSLog(@"3+4 = %d", suma(3, 4));

// Block type
typedef void (^callbackLogger)(void);

// Use as parameter
void genericLogger(callbackLogger block) {
    block();
}

File Operations

NSFileManager *fileManager = [NSFileManager defaultManager];

// Check existence
[fileManager fileExistsAtPath:@"/path/to/file.txt"];

// Copy
[fileManager copyItemAtPath:@"/src" toPath:@"/dst" error:nil];

// Compare contents
[fileManager contentsEqualAtPath:@"/file1" andPath:@"/file2"];

// Delete
[fileManager removeItemAtPath:@"/path" error:nil];

Security Analysis Patterns

1. Extract Class Information

# Basic dump
class-dump /Applications/Kindle.app/Contents/MacOS/Kindle

# With headers
class-dump -H /path/to/binary > output.h

# Recursive for frameworks
class-dump -r /path/to/framework.framework

2. Identify Sensitive Methods

Look for methods containing:

• password, credential, auth, token
• key, secret, private
• decrypt, encrypt, encode, decode
• file, path, document, home
• admin, root, sudo, privilege

3. Find File Operations

Search for NSFileManager usage and file paths in:

• fileExistsAtPath:
• copyItemAtPath:toPath:
• removeItemAtPath:
• contentsEqualAtPath:andPath:

4. Analyze Protocol Implementations

Protocols often indicate:

• Delegation patterns (potential hook points)
• Required vs optional methods (behavior variations)
• Interface contracts (expected behavior)

Compilation

# Compile Objective-C file
gcc -framework Foundation test_obj.m -o test_obj

# With clang (recommended)
clang -framework Foundation test_obj.m -o test_obj

Quick Reference

Common Pitfalls

1. Obfuscated names: Class/method names may be mangled in production binaries
2. Immutable vs Mutable: NSString/NSArray/NSDictionary are immutable by default
3. Memory management: Modern ARC handles this, but older code may use manual retain/release
4. Block captures: Blocks capture variables by reference unless specified otherwise
5. Protocol conformance: Classes must explicitly adopt protocols with <ProtocolName>

Next Steps

For deeper analysis:

• Use otool -tv <binary> to disassemble methods
• Use nm <binary> to list symbols
• Use lipo -info <binary> to check architecture
• Use codesign -dv <binary> to inspect code signature