The remote host is missing an update to libpng announced via advisory DSA 570-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20570-1

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) this problem has been fixed in version 1.0.12-3.woody.9. For the unstable distribution (sid) this problem has been fixed in version 1.0.15-8. We recommend that you upgrade your libpng packages.