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December 16, 2020

GHSL-2020-330: Unauthorized repository modification or secrets exfiltration in two akka repositories

Jaroslav Lobacevski

Summary

The alpakka-kafka/link-validator.yml and akka-grpc/link-validator.yml GitHub workflows are vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

akka/alpakka-kafka and akka/akka-grpc GitHub repositories.

Tested Version

The latest changesets 2d8216e and ff0ad29 to the date.

Details

Issue: Untrusted code is explicitly checked out and run on a Pull Request from a fork

pull_request_target was introduced to allow triggered workflows to comment on PRs, label them, assign people, etc.. In order to make it possible the triggered action runner has read/write token for the base repository and the access to secrets. In order to prevent untrusted code from execution it runs in a context of the base repository.

By explicitly checking out and running build script from a fork the untrusted code is running in an environment that is able to push to the base repository and to access secrets.

on:
  pull_request_target:
    branches: [master, main]
...
      - name: Checkout
        uses: actions/checkout@v2
        with:
          # See https://github.com/actions/checkout/issues/299#issuecomment-677674415
          ref: ${{ github.event.pull_request.head.sha }}
          fetch-depth: 100
...
      - name: sbt site
        run: sbt docs/makeSite
...

Impact

The vulnerability allows for unauthorized modification of the base repository and secrets exfiltration.

Coordinated Disclosure Timeline

Credit

This issue was discovered and reported by GHSL team member @JarLob (Jaroslav Lobačevski).

Contact

You can contact the GHSL team at securitylab@github.com, please include a reference to GHSL-2020-330 in any communication regarding this issue.