[Security-news] PSA-2012-001 - Hash DOS attack prevention with Suhosin needs a .htaccess edit

[security-news at drupal.org]

  * Advisory ID: DRUPAL-PSA-2012-001
  * Project: Drupal core [1]
  * Version: 6.x, 7.x
  * Date: 2012-01-11
  * Security risk: Less critical [2]
  * Exploitable from: Remote
  * Vulnerability: Denial of Service

-------- DESCRIPTION  
---------------------------------------------------------

PHP is vulnerable to a hash collision denial of service (DOS) [3] attack. If
an attacker can post a large amount of specifically chosen variables to the
site, a large amount of CPU time is consumed preventing service to visitors.

Many users deploy the Suhosin PHP extension [4] to limit the amount of posted
variables that will be handled by PHP, thus preventing the DOS attack.

There's an unfortunate interaction with the mbstring extension required by
Drupal to work with UTF-8. When the setting mbstring.encoding_translation is
updated via .htaccess the mbstring extension changes the PHP POST handlers so
that only every other POST variable can be handled by Suhosin.

While Suhosin will still remove half of the variables over the post.max_vars
limit, it is ultimately unsuccesful in limiting the amount of posted
variables and thus in preventing the hash collision DOS attack.

-------- VERSIONS AFFECTED  
---------------------------------------------------

All versions

-------- SOLUTION  
------------------------------------------------------------

Confirm that the master value of mbstring.encoding_translation is set to Off
via:

  * Drupal 7: Reports > Status, then More information on the PHP version
    (admin/reports/status/php)
  * Drupal 6: Administer > Reports > Status report, then follow the link on
    the PHP version (admin/reports/status/php)

Next, remove the lines from the file .htaccess in the Drupal root.

For Drupal 7.x remove the lines:
php_flag mbstring.encoding_translation off

For Drupal 6.x remove the lines:
php_value mbstring.encoding_translation off

If the master value of mbstring.encoding_translation is On, change it to Off
via PHP.ini. Contact your hosting provider if necessary.

If you do not use Suhosin, limit the amount of variables posted to your site
in another way. You should consider upgrading to PHP 5.3.9 and using its
newly introduced directive 'max_input_vars'.

Please note that setting such limits too low (whether via Suhosin or PHP) can
break processing on long forms like the permissions administration screen.

It is likely that the near-future will see an update to Suhosin, making the
procedure described in this PSA unnecessary.

See also the Drupal core [5] project page.

-------- REPORTED BY  
---------------------------------------------------------

  * Dominic Böttger

-------- CONTACT AND MORE INFORMATION  
----------------------------------------

The Drupal security team can be reached at security at drupal.org or via the
contact form at http://drupal.org/contact [6].

Learn more about the Drupal Security team and their policies [7], writing
secure code for Drupal [8], and securing your site [9].


[1] http://drupal.org/project/drupal
[2] http://drupal.org/security-team/risk-levels
[3] http://www.ocert.org/advisories/ocert-2011-003.html
[4] http://www.hardened-php.net/
[5] http://drupal.org/project/drupal
[6] http://drupal.org/contact
[7] http://drupal.org/security-team
[8] http://drupal.org/writing-secure-code
[9] http://drupal.org/security/secure-configuration