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Drupal Embedded Media Field Module XSS Vulnerability

30 November -0001

Description of Vulnerability:

Drupal (http://drupal.org) is a robust content management system (CMS) written in PHP and MySQL. The Drupal Embedded Media Field module (http://drupal.org/project/emfield) "will create fields for content types that can be used to display video, image, and audio files from various third party providers" Unfortunately the Embedded Media Field module contains an arbitrary HTML injection vulnerability (also known as cross site scripting, or XSS) due to the fact that it fails to sanitize filenames of thumbnail images before display.

Systems affected:

Drupal 6.19 with Embedded Media Field 6.x-1.25 and CCK 6.x-2.8 was tested and shown to be vulnerable

Impact

Users could inject arbitrary scripts into pages affecting other site users. This could result in administrative account compromise leading to web server process compromise. A more likely scenario would be for an attacker to inject hidden content (such as iframes, applets, or embedded objects) that would attack client browsers in an attempt to compromise site users' machines. This vulnerability could also be used to launch cross site request forgery (XSRF) attacks against the site that could have other unexpected consequences.

Mitigating factors:

In order to exploit this vulnerability the attacker must have the ability to edit content of a content type with an embedded media field. Also, many operating systems prevent the creation of files with slashes in their names so clever use of scripting without slashes is required to exploit this vulnerability.

Proof of concept:

  1. Install Drupal 6-19, CCK module, and Embedded Media Field module version 6.x-1.25
  2. Enable the Content, Embedded Media Field, Embedded Media Thumbnail and Embedded Video Field modules from ?q=/admin/build/modules
  3. Alter the default 'Story' content type at ?q=admin/content/node-type/story/fields
  4. Add a 'New Field' in the form at the bottom of this page with the label 'video' the field name 'field_video' the type 'Embedded Video' and the form element '3rd Party Video' then click the 'Save' button
  5. Configure the new video field from ?q=admin/content/node-type/story/fields/field_video
  6. Select YouTube as a content provider for convenience and be sure 'Allow custom thumbnails for this field' is checked and click 'Save field settings' button at the bottom of the form
  7. Create a new piece of story content from ?q=node/add/story entering arbitrary values. For the 'Video custom thumbnail' choose an image with a name like "<image src='no.jpg' onerror='alert("xss")'>.png" and click the 'Upload' button
  8. Observe the rendered javascript alert dialogue
  9. Click the 'Save' button so that the XSS persists to future node edits

Patch:

Applying the following patch mitigates this issue in version 6.x-1.25 

--- emfield/contrib/emthumb/emthumb.module	2010-07-19 11:12:47.000000000 -0400
+++ emfield/contrib/emthumb/emthumb.module	2010-11-04 16:10:48.000000000 -0400
@@ -157,7 +157,7 @@ function emthumb_widget_element_process(
 
     $element['emthumb']['description'] = array(
       '#type' => 'markup',
-      '#value' => '<strong>'. t('Filename:') .' </strong>'. $file['filename'],
+      '#value' => '<strong>'. t('Filename:') .' </strong>'. check_plain($file['filename']),
     );
 
     // Overwrite with an input field if custom_alt is flagged.

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