Project with partner company, during the meeting I asked them how can we secure the communication between two services. I suggested api keys, tokens. They were like nope, no need. But I asked them for their IPs to do whitelisting on our side in Nginx.

But their side, nah not even whitelisting, no tokens, no validations. If one has address, can send anything from anywhere.

How hard would it be to do at least, AT LEAST simple token validation. And they are using the very old IIS server. I think for them as long as data flows in as expected, it is fine.