> For the complete documentation index, see [llms.txt](https://gitbook-88.gitbook.io/ctf-writeup/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gitbook-88.gitbook.io/ctf-writeup/2020/2020-yang-cheng-bei/blackcat.md).
# BlackCat
```php
if(empty($_POST['Black-Cat-Sheriff']) || empty($_POST['One-ear'])){
die('谁！竟敢踩我一只耳的尾巴！');
}
$clandestine = getenv("clandestine");
if(isset($_POST['White-cat-monitor']))
$clandestine = hash_hmac('sha256', $_POST['White-cat-monitor'], $clandestine);
$hh = hash_hmac('sha256', $_POST['One-ear'], $clandestine);
if($hh !== $_POST['Black-Cat-Sheriff']){
die('有意瞄准，无意击发，你的梦想就是你要瞄准的目标。相信自己，你就是那颗射中靶心的子弹。');
}
echo exec("nc".$_POST['One-ear']);
```
音频存在伪代码
可以看出大概是将密钥再加密后用来加密输入的命令，进行强等判断，如何绕过关键点就是让环境变量`$clandestine`被加密后可控，这里用了密钥传入数组的方法，加密后''使`$clandestine`为一个定值，最终Payload：
---
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