The remote host is missing an update to wu-ftpd announced via advisory DSA 357-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20357-1

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.