December 16, 2020

GHSL-2020-272: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of lampepfl/dotty

Jaroslav Lobačevski

Summary

The scala3doc.yaml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.

Product

lampepfl/dotty GitHub repository

Tested Version

The latest changeset 3f4b101 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:
jobs:
  build:
    runs-on: ubuntu-latest
...
      - name: Checkout to PR code
        env:
          PR_NUMBER: ${{ github.event.pull_request.number }}
        run : |
          if [[ -z "${PR_NUMBER}" ]]; then
            echo Not a pull request do not need to checkout
          else
            REF=refs/pull/$PR_NUMBER/merge
            echo checking $REF
            git fetch origin $REF:$REF
            git checkout $REF
          fi     
...
      - name: Compile and test
        run: ./project/scripts/sbt scala3doc/test
...

Impact

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

Coordinated Disclosure Timeline

  • 2020-11-30: Report sent to maintainers
  • 2020-12-01: Issue resolved

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-2020-272 in any communication regarding this issue.