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December 16, 2020

GHSL-2020-317: Unauthorized repository modification or secrets exfiltration in gpuweb/cts repository

Jaroslav Lobacevski

Summary

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

Product

gpuweb/cts GitHub repository

Tested Version

The latest changeset d516cfa 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:
      - main
      - glsl-dependent
...
      - run: |
          git fetch origin ${{ github.event.pull_request.head.sha }}
          git checkout ${{ github.event.pull_request.head.sha }}
      - uses: actions/setup-node@v2-beta
        with:
          node-version: "15.x"
      - run: npm ci
      - run: |
          npm test
          sed -i '/out\//d' .gitignore
          sed -i '/out-wpt\//d' .gitignore
...

Impact

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

Coordinated Disclosure Timeline

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-2020-317 in any communication regarding this issue.