Cross-site scripting
The Forum module does not correctly handle certain arguments
obtained from the URL. By enticing a suitably privileged user
to visit a specially crafted URL, a malicious user is able to
insert arbitrary HTML and script code into forum pages. Such a
cross-site scripting attack may lead to the malicious user
gaining administrative access. Wikipedia has more information
about cross-site scripting (XSS).
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.
When an anonymous user fails to login due to mistyping his
username or password, and the page he is on contains a sortable
table, the (incorrect) username and password are included in
links on the table. If the user visits these links the password
may then be leaked to external sites via the HTTP referer.
In addition, if the anonymous user is enticed to visit the site
via a specially crafted URL while the Drupal page cache is
enabled, a malicious user might be able to retrieve the
(incorrect) username and password from the page cache.