README.md

ncrack or Cracking using LC_REEXPORT_DYLIB

In a situations where we would like only to replace the specific symbols or methods in ObjC world, we can utilize LC_REEXPORT_DYLIB load command.

This load command allows us to load a specific library; if that specific library does not have some symbols we can find them somewhere else(in exported library).

To demonstrate the usage, we can use the following code.

main.c:

#include <stdio.h>

extern void hello_from_the_library(void);
extern void to_replace(void);

int main(void)
{
    hello_from_the_library();
    to_replace();
    return 0;
}

library.c:

#include <stdio.h>

void hello_from_the_library(void)
{
    printf("Hello from dylib\n");
}

void to_replace(void)
{
    printf("I am not replaced\n");
}

Makefile:

all:
	@gcc -o libcool_lib.dylib -dynamiclib library.c
	@gcc main.c -o cool_bin -L. -lcool_lib

Running make followed with ./cool_bin gives us the following output which is expected.

$ ./cool_bin
Hello from dylib
I am not replaced

Let's now say that we want to only replace to_replace function inside the libcool_lib.dylib.

The code for that is pretty simple.

replacer.c:

#include <stdio.h>

void to_replace(void)
{
    printf("I am replaced\n");
}

Now, we cant just blindly compile this code as dynamic library, because once the application tries to load the dylib, it will fail resolving the symbol.

dyld[7150]: Symbol not found: _hello_from_the_library
  Referenced from: <339A7448-1A2C-315F-AE36-6AA56B30F1D8> /Users/demon/tools/ncrack/cool_bin
  Expected in:     <E1CAFA7D-FAF0-3330-BAE2-2EB4F0D34994> /Users/demon/tools/ncrack/libcool_lib.dylib
fish: Job 1, './cool_bin' terminated by signal SIGABRT (Abort)

To overcome this, we can build our new dylib with LC_REEXPORT_DYLIB that points to our original dylib.

But, before we compile it, let's rename the original library to _libcool_lib.dylib.

$ gcc -o libcool_lib.dylib -dynamiclib replacer.c -Xlinker -reexport_library /Users/demon/tools/ncrack/_libcool_lib.dylib

Running otool -l on this new library confirms that we have added LC_REEXPORT_DYLIB to it.

$ otool -l ./libcool_lib.dylib | grep LC_REEXPORT -A5
          cmd LC_REEXPORT_DYLIB
      cmdsize 48
         name libcool_lib.dylib (offset 24)
   time stamp 2 Thu Jan  1 01:00:02 1970
      current version 0.0.0
compatibility version 0.0.0

The only thing left is that we need to tell the binary where to actually find this new library. We can do that using install_name_tool.

Make sure to provide full paths while compiling the new library as well as when calling install_name_tool.

$ # install_name_tool -change NAME OLD NEW
$ install_name_tool -change libcool_lib.dylib \
  /Users/demon/tools/ncrack/_libcool_lib.dylib \
  /Users/demon/tools/ncrack/libcool_lib.dylib

Running the binary now shows that the binary has called our new function.

We can do the same thing with ObjC methods using method swizzling with LC_REEXPORT_DYLIB trick. Sample code is given in objc directory.