Drupal 5.2 and earlier does not properly unset variables when the input data includes a numeric parameter with a value matching an alphanumeric parameter's hash value, which allows remote attackers to execute arbitrary PHP code by invoking the drupal_eval function through a callback parameter to the default URI, as demonstrated by the _menu[callbacks][1][callback] parameter.  NOTE: it could be argued that this vulnerability is due to a bug in the unset PHP command (CVE-2006-3017) and the proper fix should be in PHP; if so, then this should not be treated as a vulnerability in Drupal.