Today we updated the Drupal 6 addthis module to 6.x-2.9 as we became aware of a potential security issue. The issue was very minor and could only be exploited by someone who already had privileged on the site, so the chance of harm being done was remote, but the update was important to maintain security.
A list of what's changed is available. Please note that even if your site makes use of addthis.com, it might not necessarily be using this Drupal module. This is because we only implement the module on sites where we need the extra functionality it provides.
If you would like more information about this issue, or would like to know if your site was affected, please get in touch using the contact details in the site footer, or via the contact page.