Masscan is a port scanner built for one thing: speed at scale. It can scan the entire IPv4 internet for a single open port in minutes, and it brings that same speed to a single organisation’s address space. Where a traditional scanner walks through hosts methodically, Masscan sprints across enormous ranges to answer one question fast , what is open?
We use it at the start of external engagements. Here is what it does and what it means when it finds an open port you did not know about.
What makes it different from Nmap
Both find open ports, but they are built for opposite priorities:
| Masscan | Nmap | |
|---|---|---|
| Priority | Raw speed, huge ranges | Depth and detail |
| Scale | Millions of IPs | Tens to thousands |
| Output | “This port is open” | Service, version, OS, scripts |
| Role | Find what is open, fast | Investigate what was found |
They are not competitors , they are sequential. Masscan sweeps a large range at speed to find open ports; Nmap then investigates those specific ports in depth. The workflow on any sizeable external engagement is Masscan to find, Nmap to understand.
The speed comes from a design choice: Masscan uses its own TCP/IP stack and transmits probes asynchronously without waiting for each reply, so it is bounded by how fast it can send packets rather than by round-trip latency.
What it is used for in a test
Masscan answers the very first question of an external assessment: what is actually exposed to the internet?
Organisations are consistently wrong about this. The answer includes not just the intended web and mail servers but the forgotten ones , a database port open to the world, a management interface that should have been firewalled, a service someone stood up for a demo and never took down, an RDP port waiting to be brute-forced.
Masscan finds all of it quickly by sweeping the full external range for open ports, which then become the targets for deeper investigation. This is why external testing starts with breadth: you cannot assess an exposed service you never knew was listening.
What its findings mean
An open port is not automatically a vulnerability, but certain findings are consistently serious:
- Databases exposed to the internet , MySQL (3306), MSSQL (1433), MongoDB (27017), Redis (6379), Elasticsearch (9200). These should almost never be internet-facing, and when they are, they are frequently unauthenticated.
- Management and remote-access interfaces , RDP (3389), VNC (5900), SSH (22) on unexpected hosts. RDP exposed to the internet is a leading ransomware entry point.
- Development and staging services , applications running with debug enabled or default credentials, exposed because a firewall rule was never applied.
- Legacy protocols , Telnet, FTP, SMB open externally, none of which belong on the public internet.
Each of these is an attack path that exists because a service was reachable when it should not have been. The fix is almost always the same: it should not be exposed at all.
How defenders see it
Masscan is fast, but at full speed it is also loud , a burst of connection attempts across many ports in a short window is an obvious signature. Detection and defence:
- Firewall and IDS logs show a spike of connections across a wide port range from one source.
- Rate-based rules flag the volume.
But as with most external findings, detection is secondary. The primary control is attack-surface reduction:
- Default-deny at the perimeter , expose only the ports that must be public, and firewall everything else.
- Maintain an external asset inventory , know every IP you own and what runs on it.
- Run your own scans regularly , attackers scan the whole internet continuously; scanning your own range before they do is the entire point of external testing.
Where it fits in an engagement
Masscan is a reconnaissance tool, the opening move of external network penetration testing. It defines the target set , the exposed ports and services , that the rest of the engagement investigates and, where warranted, exploits. It makes no judgment about whether a service is vulnerable; it establishes what is reachable, which is the necessary first step to finding out.
The short version
Masscan finds open ports across huge address ranges at extreme speed, which makes it the first tool in external reconnaissance: it tells you what is actually exposed to the internet, which is reliably more than you think. The dangerous findings , exposed databases, open RDP, forgotten services , are attack paths created by services being reachable when they should not be. The defence is a default-deny perimeter and an accurate inventory, validated by scanning your own footprint before an attacker scans it for you.
Want to know exactly what your organisation exposes to the internet right now? Mapping that is where every external penetration test begins. Scope one here.
Written by
Invadel Team
Senior penetration testers writing from real engagements — the same team that scopes, tests, and reports for our clients. About Invadel →