Summary
The checks.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.
Product
is-a-dev/register GitHub repository
Tested Version
The latest changeset 7a651a1 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]
...
- uses: actions/checkout@v2
with:
ref: ${{ github.event.pull_request.head.sha }}
fetch-depth: 0
- name: Installing dependencies
uses: borales/actions-yarn@v2.0.0
with:
cmd: install --ignore-engines
- name: Running tests
uses: borales/actions-yarn@v2.0.0
with:
cmd: 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
- 2020-11-30: Maintainers acknowledged
- 2020-11-30: 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-2020-279 in any communication regarding this issue.