This indicates an attack attempt against a heap-based Buffer Overflow vulnerability in Libgd.
The vulnerability is caused by an integer signedness error in GD Graphics Library. By sending a specially crafted G2 request to the target, a remote attacker could overflow a buffer and execute arbitrary code on a vulnerable system.
GD Library libgd 2.1.1 prior to 2bb97f407c1145c850416a3bfbcc8cf124e68a19 commit
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.