Using OAuth2 access tokens is the best practice for authenticating an API request by a resource server. As stated in the draft CNCF Zero Trust White Paper, it is recommended to verify the "audience" of the access token to prevent access tokens from being consumed by other recipients ("Token Redirect" attack). Especially in cloud-native architectures, there are many internal workloads, so it's hard for the resource owner to identify all audiences and consent for each consumption. In this case, we can adopt the OAuth WG's draft called "Transaction Tokens" (Txn-Tokens), which utilizes OAuth2 Token Exchange (RFC8693) to issue Txn-Tokens that allow downstream workloads to identify call chains. Keycloak, an IAM OSS, supports Token Exchange. Therefore, Keycloak can potentially support the Txn-Token service which issues Txn-Tokens.
In this presentation, Yoshiyuki Tabata provides an overview of Txn-Tokens and introduces how to implement Txn-Tokens with Keycloak.