What’s the best option for service-to-service auth: OAuth 2.0 vs API key?

[StevenCR]

I'm trying to decide between using OAuth 2.0 client credentials and a simple API key for a new internal service-to-service integration. The client credentials flow feels like overkill since there's no user involved, but I keep reading that API keys alone are less secure for machine-to-machine communication. I'm not sure which trade-off is right for a purely backend process.

[Jacob85]

Last project we started with a simple API key for the service that talks to our internal catalog. It was fast to wire up and we could rotate keys manually. Then we learned that a leaked key in logs or a leaked container image is painful to fix, so we moved to OAuth 2.0 client credentials. Token lifetimes helped us limit exposure, and we set up a short-lived access token with a central token endpoint. The overhead was real but manageable, and it forced us to centralize secrets and auditing. Sometimes it still feels heavier than it should.

[Charles.H]

API keys can be perfectly fine if you treat them like passwords: rotate, store in a secret manager, bind by network policy, and log every use. The downside is you can’t prove the client’s identity beyond possession, but for internal use with strong network controls, it might be enough. We kept ours behind mTLS and IP allowlists to reduce blast radius.

[Aria.W]

Maybe the real issue isn’t which flow but how you handle secrets overall. If the backend process never impersonates a user, are you really solving a security problem or just hiding it behind a token? I’ve seen teams chase tokens while their logging and rotation policies lag behind, and you still end up trusting the wrong things. Do you have a reliable plan to rotate, revoke, and audit without breaking automation?

[Luna.R]

We once debated if we should keep tokens in env vars or pull from a secret store on startup. It felt like we were over-engineering, then we realized the environment would be deployed across multiple clusters and drifted into what secure by design means. In practice we ended up with a capsule of credentials in a managed secret store and a simple policy to fetch tokens on demand; maybe the trick is not the protocol but the ops discipline.