When using Forgejo's `enable-openid-connect: true`, a URL is generated into the actions under `$ACTIONS_ID_TOKEN_REQUEST_URL` that can be used to generate a JWT for accessing third-party resources authenticated as the action executing in this server on this repo. However, the endpoint of that url (`.../idtoken`) had unintentionally missed a `return` on an internal server error, and was missing a check that the action actually had `enable-openid-connect: true` on it. As a result, it was possible to generate a JWT for accessing third-party resources from an action that wasn't expected to be generating JWTs.
In terms of real-world vulnerability, the most likely risk is that the JWT could be generated from a forked pull request. By not using the `$ACTIONS_ID_TOKEN_REQUEST_URL` and instead going directly to the `.../idtoken` endpoint, and parsing a generated JWT response that will be mixed with an error response, it's possible to retrieve a JWT in a forked pull request. It would require a slight misconfiguration on a third-party system to allow that JWT access, but it's a plausible risk.
As this is a feature in Forgejo 15 that hasn't been released, it will be fixed in-public.
## Checklist
The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. All work and communication must conform to Forgejo's [AI Agreement](https://codeberg.org/forgejo/governance/src/branch/main/AIAgreement.md). There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).
### Tests for Go changes
(can be removed for JavaScript changes)
- I added test coverage for Go changes...
- [ ] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
- [x] `make pr-go` before pushing
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
- Feature is not yet released.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12030
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>
