The remote host is missing an update to libpng3 announced via advisory DSA 571-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20571-1

Several integer overflows have been discovered by its upstream developers in libpng, a commonly used library to display PNG graphics. They could be exploited to cause arbitrary code to be executed when a specially crafted PNG image is processed. For the stable distribution (woody) these problems have been fixed in version 1.2.1-1.1.woody.9. For the unstable distribution (sid) these problems have been fixed in version 1.2.5.0-9. We recommend that you upgrade your libpng3 packages.