BRev Port 27017 — MongoDB
Port 27017 is MongoDB’s default listener. Before version 3.6 the server bound 0.0.0.0 with authentication disabled out of the box, and the result is one of the best-documented mass exposures in the industry: tens of thousands of databases readable by anyone who connected, followed by waves of automated wiping and ransom notes. Version 3.6 changed the default bind to localhost, which fixed new installations and did nothing for the deployed ones.
The persistent trap is that binding and authentication are separate. Enabling authentication with --auth still leaves the localhost exception in play — the first connection from the local machine may create the initial user — and many container images or configuration-management templates set bindIp to 0.0.0.0 for convenience without turning authentication on. Publishing a MongoDB container port is enough to reproduce the original mistake exactly.
The related ports are worth knowing so a scan makes sense: 27018 for shard members, 27019 for config servers, and 27017 for mongod and mongos. All are marked never in our reference for the same reason — they carry the same data with the same authentication decision behind them.
Port 27017 at a glance
| Field | Value | Detail |
|---|---|---|
| Port | 27017 | Registered |
| Service | MongoDB | The MongoDB wire protocol. (TCP) |
| Category | Databases | data stores and their wire protocols |
| Exposure | Never | Never expose to the internet |
Should port 27017 face the internet? Never — versions before 3.6 bound to 0.0.0.0 with no authentication, which is how tens of thousands of databases were wiped and ransomed.
Bind it to localhost or a private network. Mass scanners find these within minutes of a host appearing online.
What commonly goes wrong
The database is empty and there is a collection demanding payment
The known outcome of an exposed instance: automated tooling drops the databases, writes a ransom note collection, and often never had a copy to sell back. Restore from backup, close the port, enable authentication, and check the logs for when access began — anything the database held should be treated as disclosed.
“Connection refused” after upgrading past 3.6
The default bindIp became 127.0.0.1, so remote clients that worked before stop connecting. That is the fix working. Set bindIp to the specific private address the application uses — not 0.0.0.0 — and enable authentication in the same change, not afterwards.
Check port 27017 yourself
No browser can open a raw TCP socket, so no web page can honestly tell you whether this port is open on a remote host. These are the real commands, generated for port 27017 — swap example.com for the host you are actually testing.
Is port 27017 reachable on another host?
nc -vz example.com 27017"succeeded!" means the TCP handshake completed — the port is open. "Connection refused" is closed; a hang until timeout means filtered (a firewall dropped it silently).
nmap -Pn -p 27017 example.comThe only common tool that distinguishes open / closed / filtered. -Pn skips the ping sweep, so it still works against hosts that ignore ICMP.
Test-NetConnection example.com -Port 27017Read TcpTestSucceeded: True = open. It is built in — nothing to install.
telnet example.com 27017A blank screen or a banner means it connected. "Connection refused" means closed. Quit with Ctrl-] then `quit`.
curl -v --max-time 5 telnet://example.com:27017Handy when telnet is not installed: "Connected to …" proves the TCP handshake, without curl trying to speak HTTP.
What on this machine is holding port 27017?
sudo ss -tlnp | grep :27017Lists the listening socket and the PID/program holding it. Drop sudo and you get the socket but not the process name.
sudo lsof -i :27017The macOS answer to "address already in use". Kill it with `kill -9 <PID>` once you are sure what it is.
netstat -ano | findstr :27017The last column is the PID. Match it in Task Manager’s Details tab, or `taskkill /PID <pid> /F`.
No web page can tell you whether a port is open on a remote host
A browser cannot open a raw TCP socket — there is no API for it, by design. So a site that reports “port 3389 is OPEN on 203.0.113.10” is not checking from your browser; it is asking its own server to scan on your behalf. That works, but it means their server sees the host you asked about, scans from their address rather than yours, and answers a question about their network path, not yours.
We don't have a server here and won't pretend to. So this page does three honest things instead:
| Port | Proto | Service | What it is | Exposure | |
|---|---|---|---|---|---|
| 27017 | tcp | MongoDB | The MongoDB wire protocol.Never — versions before 3.6 bound to 0.0.0.0 with no authentication, which is how tens of thousands of databases were wiped and ransomed. | Never |
http://host:port/ and times it. A connection that is established proves a socket is open; an instant refusal usually means nothing is listening; anything else is honestly reported as “can't tell”. Each attempt gives up after 2.5s.nc -vz example.com 443
nmap -Pn -p 443 example.com
telnet example.com 443
curl -v --max-time 5 telnet://example.com:443
This page cannot answer that, and neither can any other page — from your browser. Opening a TCP connection to an arbitrary host and port is not something JavaScript is allowed to do. Every “open port checker” that gives you a green tick is running the scan on their server. Which is fine, as long as you know what you are agreeing to: they learn which host and port you were worried about, and the answer they give you is about the route from their data centre — not from the internet as your users see it.
What to do instead
Run the check from outside your own network. Copy an nmap or nc line from the card above and run it from a machine that is not behind the same router — a phone on mobile data with a terminal app, a cheap VPS, a colleague's laptop. Running it from inside the LAN tests the LAN, not the internet, and will happily say “open” while the world sees nothing.
Then check the thing that is actually blocking it, in this order: the service is bound to0.0.0.0 and not 127.0.0.1 · the host firewall (ufw, firewalld, Windows Defender Firewall) · the cloud security group or the router's port-forward rule · and finally your ISP, which commonly blocks 25, 80 and 445 on residential lines whatever you configure.
What a browser genuinely can verify
One thing: that an HTTPS service answers. If what you actually want to know is “is my site up and how fast is it”, the latency test measures real round trips to a CORS-open URL. If the name doesn't resolve, or resolves somewhere unexpected, that is a DNS problem, not a port problem — and it is the more common cause of “my port is closed” than an actual firewall.
An IP address finds the machine; a port picks which program on it gets the traffic. TCP sets up a connection first — a handshake, ordered delivery, retransmission — which is why the web, email and SSH use it. UDP just sends the packet with no handshake and no guarantee it arrives, which is why DNS, video calls and games use it: late data is worse than lost data.
That difference is why a UDP port is so much harder to test. TCP either completes a handshake or it doesn't, so a tool can tell you. A UDP service that has nothing to say to an unexpected packet stays silent — and silence looks identical to a firewall dropping it.
The three ranges
0–1023Assigned by IANA to core services. On Unix, binding one needs root (or CAP_NET_BIND_SERVICE).1024–49151Registered by vendors for their applications. Any user can bind these — which is why every dev server lives here.49152–65535Never assigned. The OS hands these out as the SOURCE port of your outgoing connections. (Linux actually uses 32768–60999 by default.)Closed is not the same as filtered
Closed means the host answered: the packet arrived, nothing is listening, and you got an immediate refusal (a TCP RST). Filtered means nothing answered at all — a firewall dropped your packet in silence, so you wait for a timeout and learn nothing. The difference matters: closed proves the host is reachable and the port is free; filtered tells you only that something between you and it is discarding traffic.nmap reports the two separately. A browser cannot distinguish them, which is why this page reports a timeout as “can't tell” rather than guessing.
The one habit worth keeping
Bind services to 127.0.0.1 instead of 0.0.0.0 unless something outside the machine genuinely needs them, and reach them over SSH or a VPN. Every database on the never-expose list above ships with that advice, and the leaks all begin the same way: a default bind address, a cloud instance with a public IP, and a scanner that found it within the hour.
The blocked-port list this page uses is the WHATWG Fetch Standard's “bad port” table — the ports every browser refuses to connect to (Chrome calls it ERR_UNSAFE_PORT). Local probing is additionally gated by Chrome's Local Network Access permission, which replaced Private Network Access and ships in Chrome 142.
FAQ
Is MongoDB secure by default now?
Safer: since 3.6 it binds to localhost, so a fresh install is not exposed. Authentication is still not on by default, so a deployment that changes bindIp without enabling auth reproduces the original problem exactly.
Should port 27017 be open to the internet?
No. Keep it on loopback or a private network, enable authentication and role-based access, and use TLS. If external access is genuinely required, put it behind a VPN — not an IP allow-list alone.
What are ports 27018 and 27019?
27018 is the default for shard members and 27019 for config servers in a sharded cluster. Same data, same exposure rules; seeing them in a scan tells you the deployment is sharded.