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December 16, 2020

GHSL-2020-286: Unauthorized repository modification or secrets exfiltration in the GitHub workflow of PureStake/moonbeam

Jaroslav Lobacevski

Summary

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

Product

PureStake/moonbeam GitHub repository

Tested Version

The latest changeset 994ad70 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.

# This is only allowing pushes on the moonbeam repo or pull request.
# In the case of pull request, the CI executes the workflow from
# the commit the **PR is merging into**. This prevents malicious attack trying
# to change the CI in the PR.
####### DO NOT CHANGE THIS !! #######
on: ["push", "pull_request_target"]

...
    if: ${{ github.repository == 'PureStake/moonbeam' }}
...
    steps:
      - name: Checkout
        if: github.event_name == 'pull_request_target'
        uses: actions/checkout@v2
        with:
          fetch-depth: 0
          ref: refs/pull/${{ github.event.pull_request.number }}/merge
...
      - name: Build Parachain Node
        run: cargo build --release --verbose --all

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-2020-286 in any communication regarding this issue.