The remote host is missing an update as announced via advisory SSA:2004-124-04.

https://secure1.securityspace.com/smysecure/catid.html?in=SSA:2004-124-04

New libpng packages are available for Slackware 9.0, 9.1, and -current to fix an issue where libpng could be caused to crash, perhaps creating a denial of service issue if network services are linked with it. More details about this issue may be found in the Common Vulnerabilities and Exposures (CVE) database: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2004-0421