iSEC Security Research reports that wu-ftpd contains an off-by-one bug in the fb_realpath function which could be exploited by a logged-in user (local or anonymous) to gain root privileges. A demonstration exploit is reportedly available.
For the current stable distribution (woody) this problem has been fixed in version 2.6.2-3woody1.
For the unstable distribution (sid) an update will be available shortly.
We recommend you upgrade your wu-ftpd package immediately.