feat: add federated identity credential client authentication - #837
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Add a `client_assertion` clientAuthMethod that authenticates to the token endpoint with a federated identity credential. Instead of signing the client assertion locally with a private key (`private_key_jwt`), the assertion JWT is issued by a trusted external identity provider (e.g. AWS STS/IAM, GitHub Actions OIDC) and supplied via a new `clientAssertion` callback. The callback is invoked on every token endpoint request so that short-lived federated tokens stay fresh. clientAuthMethod defaults to `client_assertion` when `clientAssertion` is configured, mirroring the existing `clientAssertionSigningKey` -> `private_key_jwt` behavior. The implementation is provider-agnostic and pulls in no cloud SDK dependencies. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01QuaDs4RvCnLyvX1SakbNQu
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Description
Our team was looking to implement Microsoft Entra ID / Azure AD authentication with
express-openid-connectusing federated identity credentials from AWS IAM roles. Documentation on this request type is available on Microsoft's website.In this implementation, I was trying to keep things completely agnostic so any provider can be used, whether on the issuer or resource server side. This flow is very similar to the
private_key_jwtassertion but does not require a local certificate to function.I did use Claude Opus 4.8 to help generate this - I was not sure if AI-generated contributions are allowed in this project, so feel free to close this if not. But I figured since this was valuable to us, I would contribute it back to upstream. Happy to make tweaks as you see fit as well.
Testing
We are using AWS's GetWebIdentityCommand with Outbound Identity Federation enabled, and the issuer added in our Entra ID account.
Checklist