Coordinated Disclosure Timeline
- 2020-11-30: Issue reported to the maintainer.
- 2020-11-30-2021-02-23: Attempted to contact maintainers multiple times.
- 2021-02-28: Disclosure deadline reached.
- 2021-03-02: Public issue created.
- 2021-12-29: Maintainer replies that the workflow has been disabled.
Summary
The auto_merge.yml GitHub workflow is vulnerable to unauthorized modification of the base repository or secrets exfiltration from a Pull Request.
Product
BitByte-TPC/first-bit GitHub repository
Tested Version
The latest changeset a43fb3c 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:
paths:
- 'public/directory/**.json'
...
uses: actions/checkout@v2
with:
ref: ${{github.event.pull_request.head.ref}}
repository: ${{github.event.pull_request.head.repo.full_name}}
- name: Yarn install
uses: actions/setup-node@v1
- run: yarn install
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-2020-313 in any communication regarding this issue.