skip to content
Back to GitHub.com
Home Bounties CodeQL Research Advisories Get Involved Events
March 8, 2021

GHSL-2020-273: Unauthorized repository modification or secrets exfiltration in a GitHub workflow of numworks/epsilon

Jaroslav Lobacevski

Coordinated Disclosure Timeline

Summary

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

Product

numworks/epsilon GitHub repository

Tested Version

The latest changeset 110f333 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]
...
      - name: Checkout PR head
        uses: actions/checkout@v2
        with:
          ref: ${{ github.event.pull_request.head.sha }}
          path: head
      - name: Build head
        run: make -j2 -C head epsilon.elf
...

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-2020-273 in any communication regarding this issue.