a more modern, zero-trust solution like mTLS authentication

That makes sense, mTLS is great. Some services like Google Cloud SQL are really good about support for it. https://cloud.google.com/sql/docs/mysql/configure-ssl-instan...

It's not quite a zero-trust solution though due to the CA chain of trust.

mTLS is security at a different layer though than IP source whitelisting. I'd say that a lot of companies we spoke to would want both as a defense-in-depth measure. Even with mTLS, network whitelisting is relevant. If your certificate were to be exposed for instance, an attacker would still need to be able to forge a source IP address to start a connection.

I'd put it in the zero-trust category if the server (or owner of the server, etc) is the issuer of the client certificate and the client uses that certificate to authenticate itself, but I'll admit this is a pedantic point that adds nothing of substance. The idea being that you trust your issuance of the certificate and the various things that can be asserted based on how it was issued (stored in TPM, etc), rather than any parameter that could be controlled by the remote party.

If mTLS is combined with outbound connections, then IP source whitelisting is irrelevant; the external network cannot connect to your resources.

This (and more) is exactly what we (I work on it) built with open source OpenZiti, a zero trust networking platform. Bonus points, it includes SDKs so you can embed ZTN into the serverless function, a colleague demonstrated it with a Python workload on AWS - https://blog.openziti.io/my-intern-assignment-call-a-dark-we....

JWT/OIDC, where the thing you're authenticating to (like MongoDB Atlas) trusts your identity provider (AWS, GCP, Modal, GitLab CI). It's better than mTLS because it allows for more flexibility in claims (extra metadata and security checks can be done with arbitrary data provided by the identity provider), and JWTs are usually shorter lived than certificates.

We have a native OIDC integration at Modal, as well! Every container gets a token. https://modal.com/docs/guide/oidc-integration

Based.

Awesome, great for you. OIDC/JWT for cross-stuff auth should become the norm.

How do you allow a driver using that exactly?

A db connection driver? You pass the JWT as the username/password which contains the information about your identity and is signed by the identity provider that the party you're authenticating to has been configured to trust.

Or, you use a broker like Vault to which you authenticate with that JWT, and which generates a just in time ephemeral username/password for your database, which gets rotated at some point.