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February 3, 2021

GHSL-2020-280: Arbitrary code execution in deislabs/akri workflows

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

Multiple workflows are vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

deislabs/akri GitHub repository

Tested Version

The latest changeset f023a3d 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.

For example check-rust.yml:

on:
...
  pull_request_target:
    branches: [ main ]
    paths:
    - .github/workflows/check-rust.yml
    - '**.rs'
    - '**/Cargo.toml'
    - '**/Cargo.lock'
    - build/setup.sh
...
    steps:
    - name: Checkout the merged commit from PR and base branch
      uses: actions/checkout@v2
      if: github.event_name == 'pull_request_target'
      with:
        # pull_request_target is run in the context of the base repository
        # of the pull request, so the default ref is master branch and
        # ref should be manually set to the head of the PR
        ref: refs/pull/${{ github.event.pull_request.number }}/head
...
    - name: Install linux requirement
      run: ./build/setup.sh
...

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-2020-280 in any communication regarding this issue.