Hello hackers, let’s solve another CTF from category easy. This challenge is about format string vulnerability

First of all I look what type of file is it

% file racecar
racecar: ELF 32-bit LSB pie executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=c5631a370f7704c44312f6692e1da56c25c1863c, not stripped

So I started Ghidra and look at functions there Racecar functions

There are couple of interesting functions with various name and after little bit of time looking at the functions I found the right one Racecar car_menu

As you can see, there is a part where the file flag.txt is opened and the content of file is read to memory. Also you can write any message, which is printed then.

    __format = (char *)malloc(0x171);
    __stream = fopen("flag.txt","r");
    if (__stream == (FILE *)0x0) {
      printf("%s[-] Could not open flag.txt. Please contact the creator.\n",&DAT_00011548,puVar5);
                    /* WARNING: Subroutine does not return */
        "\n\x1b[3mThe Man, the Myth, the Legend! The grand winner of the race wants the whole world to know this: \x1b[0m"

So directly to the point I printed stack with %08x.. Racecar final

.. and then convert hex values to ascii, of course don’t forget about little endian. Now we have a flag and finaly we can take a 🍺 and enjoy it. cheers!