An attack surface is the total set of points, systems, accounts, connections, and devices, where an attacker could try to gain entry to or affect an organization's environment. The larger and less visible the attack surface, the more opportunities an attacker has.
What the attack surface is
The attack surface is a way of describing everything an attacker could potentially target. It is the sum of all the points of exposure: every internet-facing server, every employee account that could be phished, every piece of software with a potential vulnerability, every remote-access service, every connected device, and every third-party connection into the environment. If an attacker could conceivably use something as a way in or a way to cause harm, it is part of the attack surface.
The concept is useful because it reframes security around the attacker's perspective. Rather than asking only what are we protecting, it asks where could someone get in, and it makes clear that defense has to account for every exposed point, not just the obvious ones. A single overlooked server, forgotten account, or unmonitored device is enough, because an attacker only needs one way in. Managing the attack surface, knowing what it includes, reducing it where possible, and monitoring all of it, is a foundational part of security.
Why the attack surface keeps growing
A persistent challenge is that the attack surface of almost every organization is expanding over time, driven by the same digital transformation that delivers so much value. Every new application, cloud service, remote-access arrangement, integration with a partner, and connected device adds another point of potential exposure. The more connected and capable an organization becomes, the larger its attack surface grows, which is why security has to keep pace with digitization rather than being treated as a one-time project.
This growth is especially pronounced in healthcare. A modern hospital runs an enormous and varied attack surface: traditional IT systems, the electronic health record and its many integrations, cloud services, remote access for staff and vendors, a web of third-party suppliers, and, above all, thousands of connected medical devices. Each of these is a potential entry point, and the sheer scale and diversity make a hospital's attack surface one of the hardest to fully see and defend in any sector.
The invisible part of the attack surface
The most dangerous part of an attack surface is the part an organization cannot see. You cannot protect, monitor, or reduce exposure on something you do not know exists or cannot observe, and attackers specifically seek out these blind spots. In a hospital, the largest invisible region of the attack surface is the connected medical device fleet. These devices, infusion pumps, monitors, imaging systems, cannot run security agents, so conventional endpoint tools are blind to them, which means a large majority of the hospital's real attack surface sits in a place its primary defenses cannot see.
This is the heart of why agentless, network-level visibility matters so much. It brings the invisible part of the attack surface into view. An organization that can discover and watch every connected device, not just the ones that can host software, finally has a complete picture of where it is exposed rather than the comfortable but misleading subset that happens to run an agent. Shrinking the invisible attack surface, by seeing and protecting the devices that were previously dark, is one of the most impactful things a device-heavy organization can do.
Managing and reducing the attack surface
Good attack-surface management follows a logical sequence. First, discover everything, achieve a complete, continuously updated inventory of all systems, accounts, connections, and devices, including the unmanaged ones. Second, reduce where possible, by closing unnecessary internet-facing services, removing unused accounts, segmenting the network so a compromise cannot reach everything, and decommissioning what is not needed. Third, monitor all of it, so that exposure and attacks are visible across the whole surface rather than just part of it. And fourth, prioritize, focusing effort on the points that are both exposed and genuinely reachable or exploitable, rather than treating every item as equally urgent.
For a resilient organization, attack-surface management works hand in hand with the assumption that some attack will eventually succeed somewhere on that surface. Reducing and monitoring the surface lowers the odds and the opportunities, while containment, segmentation, and recovery ensure that a breach at any one point does not become a breach of everything. The two mindsets together, shrink the surface and prepare for the inevitable, are what a mature security posture looks like, and they are especially vital in environments like hospitals where the surface is vast and much of it cannot defend itself.
Internal versus external attack surface
It helps to distinguish two parts of the attack surface, because they call for different defenses. The external attack surface is everything an attacker can see and reach from the outside: internet-facing servers, remote-access gateways, public web applications, cloud services, and the accounts that can be targeted by phishing. This is where intrusions usually begin, so hardening it, closing unnecessary exposure, patching internet-facing systems, enforcing strong authentication, directly reduces the chance of an attacker getting a foothold at all.
The internal attack surface is everything an attacker can reach once they are inside: the systems, devices, and accounts a foothold can pivot to. This is where the damage is actually done, through lateral movement toward high-value systems, and it is where a flat, poorly segmented network becomes a liability. A large internal attack surface means that a single external breach can quickly become a whole-network compromise. Reducing it through segmentation and least privilege, so that an attacker who gets in finds little they can reach, is just as important as defending the perimeter. In a hospital, the internal surface is dominated by the connected device fleet, which is why bringing those devices into view and controlling what they can communicate with is central to shrinking the internal surface that matters most.
See also
Sources
- CISA: attack surface and asset management guidance
- Claroty: State of CPS Security 2025 — Healthcare Exposures
- NIST: Cybersecurity Framework (identify function)
Definitions are drawn from the cited sources and were current as of publication. Content was rephrased for compliance with source licensing.