The remote Debian host is missing a security-related update.
Holger Fuhrmannek discovered that missing input sanitising in the Graphite font rendering engine could result in the execution of arbitrary code.
Upgrade the iceweasel packages. For the oldstable distribution (wheezy), this problem has been fixed in version 38.6.1esr-1~deb7u1. For the stable distribution (jessie), this problem has been fixed in version 38.6.1esr-1~deb8u1.