CVE-2020-25867 : SoPlanning Sharing Key Bypass

Timeline

• 26/06/2020 : This vulnerability report was sent to the vendor
• 21/07/2020 : The vulnerability is fixed

Risks

The sharing key system is vulnerable to a PHP Type Juggling attack, which allows an attacker to access the content of the calendar without knowing the sharing key.

Requirements

To be vulnerable, the calendar sharing key must be enabled.

Once this sharing is enabled, a user can access the calendar without authentication by using the sharing key.

Of course, if the key is false, the access is denied.

Exploitation

The key validation is in the file “includes/header.inc”

if ( CONFIG_SOPLANNING_OPTION_ACCES == 2 && isset($_GET['public']) && isset($_GET['cle']))
{
  if (strcmp($_GET['cle'],CONFIG_SECURE_KEY)==0)
  {
    $_SESSION['public']=1;
    $_SESSION['user_id']='publicspl';
  }
}

The strcmp function is used with a non-strict comparison. The result of this comparison depends on the following : PHP type comparison tables

The result of the strcmp function is described in the documentation : [strcmp]‘https://www.php.net/manual/en/function.strcmp.php)

Returns < 0 if str1 is less than str2; > 0 if str1 is greater than str2, and 0 if they are equal.

This function return “NULL” if one of the parameters is an array.

Following the PHP type comparison tables, the non-strict comparison between “NULL” and 0 is true

Thus, if $_GET[‘cle’] is an array, the condition result is true and the access is authorized.

This link will define $_GET[‘cle’] as an array : https://demo.soplanning.org/planning.php?public=1&cle[]=

The access is granted as “Invité”

Fix

To fix this vulnerability, a strict comparison can be used:

if ( CONFIG_SOPLANNING_OPTION_ACCES == 2 && isset($_GET['public']) && isset($_GET['cle']))
{
  if (strcmp($_GET['cle'],CONFIG_SECURE_KEY)===0)
  {
    $_SESSION['public']=1;
    $_SESSION['user_id']='publicspl';
  }
}

Or the comparison can be done without the strcmp function.

if ( CONFIG_SOPLANNING_OPTION_ACCES == 2 && isset($_GET['public']) && isset($_GET['cle']))
{
  if ($_GET['cle'] === CONFIG_SECURE_KEY)
  {
    $_SESSION['public']=1;
    $_SESSION['user_id']='publicspl';
  }
}