Improper Input Validation
Drupal 6.x before 6.16 and 5.x before version 5.22 does not properly block users under certain circumstances. A user with an open session that was blocked could maintain their session on the Drupal site despite being blocked.
CWE-20 - Improper Input Validation
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.