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February 3, 2021

GHSL-2020-332: Arbitrary code execution in a2o/snoopy workflow

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

The code-qa-sonarcloud.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

a2o/snoopy GitHub repository

Tested Version

The latest changeset 37333bf 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
    paths-ignore:
      - '.github/*.md'
      - '.github/workflows/**'
      - '.gitignore'
      - 'ChangeLog'
...
      # In the PR-related operation mode, unlike regular github's CI workflows (where
      # the workflow operates on a (preview) merge commit (as if PR was merged into the base
      # branch already), we're operating on PR's HEAD (last commit of the PR) itself here.
      - name: Checkout (preview) merge commit for PR ${{ github.event.pull_request.number }}
        uses: actions/checkout@v2
        with:
          fetch-depth: 0   # Shallow clones should be disabled for a better relevancy of analysis
          repository: ${{github.event.pull_request.head.repo.full_name}}
          ref: ${{github.event.pull_request.head.ref}}
        if: ${{ github.event_name == 'pull_request_target' }}
...
      ### Install build environment tools + unzip
      #
      - run: ./dev-tools/install-dev-software.sh
      - run: DEBIAN_FRONTEND=noninteractive apt-get install -y unzip
...

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-2020-332 in any communication regarding this issue.