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March 8, 2021

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

Jaroslav Lobacevski

Coordinated Disclosure Timeline


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


numworks/epsilon GitHub repository

Tested Version

The latest changeset 110f333 to the date.


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
          ref: ${{ github.event.pull_request.head.sha }}
          path: head
      - name: Build head
        run: make -j2 -C head epsilon.elf


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


This issue was discovered and reported by GHSL team member @JarLob (Jaroslav Lobačevski).


You can contact the GHSL team at, please include a reference to GHSL-2020-273 in any communication regarding this issue.