Basic cracking a program using Cutter

1. Install Cutter from this link Cutter Radare.

2. Create a simple program called vulnerable_license.c.

    #include <string.h>
    #include <stdio.h>
   
    int main(int argc, char *argv[]) {
        if(argc==2) {
                printf("Checking License: %s\n", argv[1]);
                if(strcmp(argv[1], "AABB-CCDD-20-OK")==0) {
                        printf("Access Granted!\n");
                } else {
                        printf("WRONG!\n");
                }
            } 
        else {
                printf("Usage: <key>\n");
        }
        return 0;
    }
   

   This program basically check the correct license to get the access of the program.

3. Then, compile the program using default gcc option.

      gcc vulnerable_license.c -o vulnerable_license
   

    # Example: if your input is "AABB-CCDD-20-OK" you get the access, else you get "WRONG!".
   
    $ ./vulnerable_license AABB-CCDD-21-MM 
    Checking License: AABB-CCDD-21-MM
    WRONG!
   
    $ ./vulnerable_license AABB-CCDD-20-OK 
    Checking License: AABB-CCDD-20-OK
    Access Granted!
   

4. Open your Cutter app and load the compiled file before and make sure to checked the write mode.

   

   

5. You will look something like this.

   We interested in main function, because we see there's a checker, so we need to bypass the checker by changing the flow of the program.

   

   From the picture above, there's some mechanism to change the flow by using reverse jump in Cutter. We tend to change the flow of result test eax eax, by default if the result not zero or the string is not same it will jump to "WRONG!" so we need to reverse the logic, by allowing the wrong string to grant the access.

6. You need to click the jne 0x11b9 in my case, other computers maybe different. Edit -> Reverse Jump. The new change will similar like this.

   

   By runtime the flow of the program is changed.

7. Save the changes and try to run the program again.

    $ ./vulnerable_license AABB-CCDD-21-MM 
    Checking License: AABB-CCDD-21-MM
    Access Granted!
   
    $ ./vulnerable_license AABB-CCDD-20-OK
    Checking License: AABB-CCDD-20-OK
    WRONG!
   

Source: Patch Binaries with Cutter