Binary Gauntlet 1

Problem

Okay, time for a challenge. gauntlet nc mercury.picoctf.net 32853

• Program

Solution

1. Decompile the binary using Ghidra:

   main function:

   undefined8 main(void)
   
   {
       char local_78 [104];
       char *local_10;
       
       local_10 = (char *)malloc(1000);
       printf("%p\n",local_78);
       fflush(stdout);
       fgets(local_10,1000,stdin);
       local_10[999] = '\0';
       printf(local_10);
       fflush(stdout);
       fgets(local_10,1000,stdin);
       local_10[999] = '\0';
       strcpy(local_78,local_10);
       return 0;
   }

2. Alright, so same program as "Binary Gauntlet 0" except the flag is not printed on a crash and the memory address of local_78 is printed at the beginning of the program.

3. We can write some shellcode to local_78, pad out to the return address, and overwrite the return address with the address of local_78 that is printed at the beginning.

4. Run the solution script and then run cat flag.txt to get the flag.

Flag

c6e16a1b4182c2801ed657d4c482af88