The upload module in Drupal 5.x before 5.23 and 6.x before 6.18 does not properly support case-insensitive filename handling in a database configuration, which allows remote authenticated users to bypass the intended restrictions on downloading a file by uploading a different file with a similar name.
http://www.securityfocus.com/bid/42391
http://www.debian.org/security/2010/dsa-2113
http://marc.info/?l=oss-security&m=128440896914512&w=2