Do people usually run Mongo in a mode that allows unauthenticated calls? I don’t know anything about Mongo. This just seems surprising.
Its default is to only take connections that are local, usually I have my mongo clients SSH into a mongo server as opposed to opening up the port to the internet. Some Mongo users / collections are very open by default.
It has been a minute since I used Mongo for production grade projects, so some things could have changed since then.
No, but it's pretty common IME to create an Atlas cluster that has internet-wide access (0.0.0.0/0) when testing and forgetting to turn this off. According to https://jira.mongodb.org/browse/SERVER-115508, this affects unauthenticated ops. Based on the repro code itself, it looks like this happens way before authentication is checked for the corresponding OP at the OP_MSG decoding level.
So if you're using Atlas, check that your Cluster has auto upgraded already. If you're using 0.0.0.0/0, stop doing that and prefer a limited IP address range and even better, use VPC Peering or other security/network boundary features.