MongoDB's default port, registered to MongoDB, Inc. since 2019.
27017 is the default listening port for mongod and mongos; sharded and config servers use 27018 and 27019. Access control is not enabled by default, so a server started without --auth and bound beyond localhost hands full read/write to anyone who connects. Point a browser at it and MongoDB answers over HTTP with a note that you are hitting the native driver port, which makes it easy to fingerprint.
Do not expose
MongoDB's security checklist calls for enabling access control and restricting net.bindIp to trusted clients — an exposed 27017 is the classic ransomed-database scan hit.
$ curl -s http://127.0.0.1:27017/Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.