The remote host is missing an update to libgd1 announced via advisory DSA 601-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20601-1

More potential integer overflows have been found in the GD graphics library which weren't covered by our security advisory DSA 589. They could be exploited by a specially crafted graphic and 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.woody4. For the unstable distribution (sid) these problems will be fixed soon. We recommend that you upgrade your libgd1 packages.