Hacking Warhead

Hacking Warheads are a variant of ECM missiles supposed to hack incoming missiles, using the fact that each second of flight prolonges the time the enemy ships needs to communicate new orders to the salvo. This limits the defenses to those fully automated, which makes a hacking warhead AI pretty evenly matched with the incoming missile computers.
On the other hand, it is a weapon heavily dependent on the technological level of both attacker and defender, and amount of knowledge their programmers have about the other side's missile guidance and computer systems. Even then, the effects are heavily limited, which doesn't stop many outnumbered forces hoping to go into artillery range for employing some of them in their defensive missile salvoes.
They are packed into a missile because hacking into the enemy missiles takes a while (and this prolongs the time the hacking warhead has to work). Its effects are best when the enemy salvo is fired on a long range, when the missiles with hacking warheads can decelerate, stop, and then accelerate backwards, spending as much time as possible near the enemy missiles.