web29

源码：

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 <?php

/*
# -*- coding: utf-8 -*-
# @Author: h1xa
# @Date:   2020-09-04 00:12:34
# @Last Modified by:   h1xa
# @Last Modified time: 2020-09-04 00:26:48
# @email: h1xa@ctfer.com
# @link: https://ctfer.com

*/

error_reporting(0);
if(isset($_GET['c'])){
    $c = $_GET['c'];
    if(!preg_match("/flag/i", $c)){
        eval($c);
    }
    
}else{
    highlight_file(__FILE__);
}

分析源码得知过滤了字符串flag

system(“ls”)命令执行成功，然后就是要绕过获取flag

通过字符串拼接的方法进行绕过