The remote host is missing an update to libgd announced via advisory DSA 589-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20589-1

infamous41md discovered several integer overflows in the PNG image decoding routines of the GD graphics library. This could lead to the execution of arbitrary code on the victim's machine. For the stable distribution (woody) these problems have been fixed in version 1.8.4-17.woody3 of libgd1 and in version 2.0.1-10woody1 of libgd2. For the unstable distribution (sid) these problems will be fixed soon. We recommend that you upgrade your libgd1 packages.