GHSL-2023-101: Actions command injection in the CI workflow of zcash/zcash

Coordinated Disclosure Timeline

2023-05-04: Report sent to security@z.cash
2023-05-04: Fix commit is pushed

Summary

The zcash/zcash repository is vulnerable to a command injection in Actions, allowing an attacker to take over the contents of the repository and leak secrets.

Product

Zcash

Tested Version

v5.5.0

Details

Actions command injection in `checks.yml` (`GHSL-2023-101`)

The checks.yml workflow is triggered on pull_request_target (i.e., when a Pull Request against the base repository is created or updated), which also covers the case when a Pull Request originates from a fork.

Taking the above into account, this workflow runs the following step with data controlled by said fork (${{ github.head_ref }} – the name of the fork’s branch), allowing an attacker to take over the GitHub Runner and run custom commands.

- name: Ensure branch contains necessary commits for Tekton CI
  id: tekton
  # https://github.com/zcash/zcash/pull/6358
  run: git merge-base --is-ancestor 267ddf8efe36cc799c4c31772a8883ea332ef55b ${{ github.head_ref }}

This issue was found by the Expression injection in Actions CodeQL query.

Proof of Concept

Fork the zcash/zcash repository, whose name we will refer as username/zcash-fork.
Create a branch in username/zcash-fork with the name master;echo${IFS}"hello";#.
Push any change to the former branch.
Create a pull request from username/zcash-fork:master;echo${IFS}"hello";# to zcash/zcash:master.
Check the workflow runs for the new Actions run that will execute the command echo "hello".

Impact

This issue may lead to stealing workflow secrets and modification of the repository.

Resources

Credit

This issue was discovered and reported by GHSL team member @jorgectf (Jorge Rosillo).

Contact

You can contact the GHSL team at securitylab@github.com, please include a reference to GHSL-2023-101 in any communication regarding this issue.