Drupal 6.x before 6.13 does not prevent users from modifying user signatures after the associated comment format has been changed to an administrator-controlled input format, which allows remote authenticated users to inject arbitrary web script, HTML, and possibly PHP code via a crafted user signature.
Improper Control of Generation of Code The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.