The remote host is missing an update as announced via advisory SSA:2003-259-03.

https://secure1.securityspace.com/smysecure/catid.html?in=SSA:2003-259-03

Upgraded WU-FTPD packages are available for Slackware 9.0 and - -current. These fix a problem where an attacker could use a specially crafted filename in conjunction with WU-FTPD's conversion feature (mostly used to compress files, or produce tar archives) to execute arbitrary commands on the server. In addition, a MAIL_ADMIN which has been found to be insecure has been disabled. We do not recommend deploying WU-FTPD in situations where security is required.