Update: According to Drupal security team member Heine Deelstra, there is no URL manipulation or JavaScript contained in the link itself involved in the exploitation of the vulnerability. The vulnerability occurs if a user with 'administrator blocks' permission has added JavaScript to a block description on the block administration page. The impact is low since not that many sites using context have role seperation between block admins and other admins. Only a small subsection of the sites using the context release candidate are affected.
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