GitHub Actions needs OIDC audience constraints

GitHub Actions' current mechanism for granting OIDC identities to workflows presents a notable security vulnerability. Unlike GitLab, which requires the OIDC audience (aud) to be statically declared, GitHub requires dynamic audience selection during runtime. This design decision allows a job to request ID tokens with any audience, enabling an attacker to potentially misapply stolen credentials to other services.

The critical claim in an OIDC ID token is the audience (aud), which prevents token acceptance by unauthorized services. However, by allowing dynamic audience control, GitHub inadvertently empowers attackers to exploit this weakness. This issue is particularly concerning given that many jobs running on GitHub Actions execute third-party code, potentially leading to the creation of ID tokens with unintended audiences.

While Trusted Publishing's design mitigates this risk by incorporating workflow names into machine identities, such a constraint may not suit all use cases. Therefore, GitHub could introduce a mechanism to constrain the audiences for which jobs can request ID tokens, thereby enhancing security without impeding legitimate workflows.