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February 3, 2021

GHSL-2020-243: Arbitrary code execution in preslavmihaylov/todocheck workflow

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

The ‘master.yaml’ GitHub workflow is vulnerable to arbitrary code execution.

Product

preslavmihaylov/todocheck GitHub repository

Tested Version

The latest changeset 9a28aee 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

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        with:
          ref: ${{github.event.pull_request.head.ref}}
          repository: ${{github.event.pull_request.head.repo.full_name}}
      - name: Build binary
        run: make build
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
        with:
          ref: ${{github.event.pull_request.head.ref}}
          repository: ${{github.event.pull_request.head.repo.full_name}}
      - name: Run tests
        env:
          TODOCHECK_ENV: "ci"
          TESTS_GITHUB_APITOKEN: ${{ secrets.TESTS_GITHUB_APITOKEN }}
        run: make 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-2020-243 in any communication regarding this issue.