February 3, 2021

GHSL-2020-274: Arbitrary code execution in v8/v8.dev workflow

Jaroslav Lobačevski

Coordinated Disclosure Timeline

  • 2020-11-30: Report sent to maintainer.
  • 2020-11-30: Report acknowledged.
  • 2020-12-02: Issue resolved.

Summary

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

Product

v8/v8.dev GitHub repository

Tested Version

The latest changeset 3954605 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]
...
      - uses: actions/checkout@v2
        with:
          ref: ${{github.event.pull_request.head.ref}}
          repository: ${{github.event.pull_request.head.repo.full_name}}
      - run: npm install
      - run: npm run lint
      - run: npm run build
...

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-2020-274 in any communication regarding this issue.