[SWPUCTF 2022 新生赛]funny_php

进入靶场环境

<?php
    session_start();
    highlight_file(__FILE__);
    if(isset($_GET['num'])){
        if(strlen($_GET['num'])<=3&&$_GET['num']>999999999){
            echo ":D";
            $_SESSION['L1'] = 1;
        }else{
            echo ":C";
        }
    }
    if(isset($_GET['str'])){
        $str = preg_replace('/NSSCTF/',"",$_GET['str']);
        if($str === "NSSCTF"){
            echo "wow";
            $_SESSION['L2'] = 1;
        }else{
            echo $str;
        }
    }
    if(isset($_POST['md5_1'])&&isset($_POST['md5_2'])){
        if($_POST['md5_1']!==$_POST['md5_2']&&md5($_POST['md5_1'])==md5($_POST['md5_2'])){
            echo "Nice!";
            if(isset($_POST['md5_1'])&&isset($_POST['md5_2'])){
                if(is_string($_POST['md5_1'])&&is_string($_POST['md5_2'])){
                    echo "yoxi!";
                    $_SESSION['L3'] = 1;
                }else{
                    echo "X(";
                }
            }
        }else{
            echo "G";
            echo $_POST['md5_1']."\n".$_POST['md5_2'];
        }
    }
    if(isset($_SESSION['L1'])&&isset($_SESSION['L2'])&&isset($_SESSION['L3'])){
        include('flag.php');
        echo $flag;
    }

    
?> 

这个题无非就是几个题目拼在一起

先看第一部分

if(isset($_GET['num'])){
        if(strlen($_GET['num'])<=3&&$_GET['num']>999999999){
            echo ":D";
            $_SESSION['L1'] = 1;
        }else{
            echo ":C";
        }
    } 

这个意思要我们找到一个比3小又要比999999999大的数，常理来说肯定是找不到的，但在strlen函数里面，他会匹配字母前的数字，比如1e9，前面1后面接字母，因此strlen函数会认为1e9=1，而1e9的真实值又比999999999大，所以用科学计数法绕过

payload

?num=1e9

接下来看第二部分

if(isset($_GET['str'])){
        $str = preg_replace('/NSSCTF/',"",$_GET['str']);
        if($str === "NSSCTF"){
            echo "wow";
            $_SESSION['L2'] = 1;
        }else{
            echo $str;
        }
    } 

题目说将NSSCTF替换成空，双写绕过不就行了

payload

?str=NSSCNSSCTFTF

接下来是第三部分

if(isset($_POST['md5_1'])&&isset($_POST['md5_2'])){
        if($_POST['md5_1']!==$_POST['md5_2']&&md5($_POST['md5_1'])==md5($_POST['md5_2'])){
            echo "Nice!";
            if(isset($_POST['md5_1'])&&isset($_POST['md5_2'])){
                if(is_string($_POST['md5_1'])&&is_string($_POST['md5_2'])){
                    echo "yoxi!";
                    $_SESSION['L3'] = 1;
                }else{
                    echo "X(";
                }
            }
        }else{
            echo "G";
            echo $_POST['md5_1']."\n".$_POST['md5_2'];
        }
    } 

这一部分的代码看上去很长，不过就是个纸老虎，关键在

$_POST['md5_1']!==$_POST['md5_2']&&md5($_POST['md5_1'])==md5($_POST['md5_2'])

这个不就是个md5弱比较嘛

payload

md5_1=QNKCDZO&md5_2=240610708