A beachhead is an initially compromised node within a network from which an attacker intends to launch further attacks on the rest of the network. The term is taken from the military, in which it refers to the objective of a military force creating a secure position when attempting to take an enemy shore from which it can then make further moves inshore.
In network security, a weakness in a single component is the best way to break into an otherwise well protected structure. Attackers are therefore more likely to spend time looking for any major vulnerabilities that they can exploit relatively quickly, rather than spend a lot of time trying to breach well defended nodes. Once the perimeter is breached, they can then carry their attack to the rest of the network.
Last updated: 8 November 2017