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February 3, 2021

GHSL-2020-328: Arbitrary code execution in GoogleCloudPlatform/microservices-demo workflow

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

The ci-pr.yaml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

GoogleCloudPlatform/microservices-demo GitHub repository

Tested Version

The latest changeset 08adad9 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 should allow secret access to fork PRs (deploy tests)
  pull_request_target:
    branches:
      - master
...
    - uses: actions/checkout@v2
      with:
        ref: ${{github.event.pull_request.head.ref}}
        repository: ${{github.event.pull_request.head.repo.full_name}}
    - name: Go Unit Tests
      timeout-minutes: 10
      run: |
...
          go test
...

Impact

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

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-2020-328 in any communication regarding this issue.