ftpd, as used by Gentoo and Debian Linux, sets the gid to the effective uid instead of the effective group id before executing /bin/ls, which allows remote authenticated users to list arbitrary directories with the privileges of gid 0 and possibly enable additional attack vectors.
http://www.securityfocus.com/archive/1/460742/100/0/threaded
http://securityreason.com/securityalert/2330
http://packages.qa.debian.org/l/linux-ftpd/news/20061125T181702Z.html