User signatures have no separate input format, they use the format of the comment with which they are displayed. A user will no longer be able to edit a comment when an administrator changes the comment's input format to a format that is not accessible to the user. However they will still be able to modify their signature, which will then be processed by the new input format.
If the new format is very permissive, via their signature, the user may be able to insert arbitrary HTML and script code into pages or, when the PHP filter is enabled for the new format, execute PHP code.
This issue affects Drupal 6.x only.
Vendor Status:
Drupal issued an update for this vulnerability
