github-actions-hardening

GitHub Actions Hardening

A focused security reviewer for GitHub Actions workflows. It reasons about the Actions-specific threat model — where trust boundaries live in trigger types, token scopes, and string interpolation — rather than the application-code vulnerabilities a general security scanner looks for. Most workflow risks are invisible to language linters because the dangerous code is the YAML itself and the way GitHub expands ${{ }} expressions into a shell before your script runs.

When to Use This Skill

Use this skill when the request involves:

• Reviewing, auditing, or hardening any file under .github/workflows/
• Authoring a new workflow and wanting it secure by default
• A workflow that uses pull_request_target, workflow_run, or issue_comment triggers
• Questions about GITHUB_TOKEN permissions or the permissions: key
• Pinning actions to commit SHAs vs tags vs branches
• Handling untrusted input (issue titles, PR bodies, branch names, commit messages) in run: steps
• OIDC / cloud authentication from Actions, or secret handling in CI
• Self-hosted runners on public repositories
• Any request like "is this workflow safe?", "secure my CI", or "review this GitHub Action"

The Core Insight

In a workflow, ${{ <expr> }} is expanded by the runner into the script before the shell executes it. So a step like:

- run: echo "Title: ${{ github.event.issue.title }}"

is not passing a variable — it is pasting attacker-controlled text directly into your shell command. An issue titled "; <attacker-command> # is concatenated into the script and executed. This single mechanism is the most common real-world Actions vulnerability, and models routinely generate it. Treat every ${{ }} that contains data an outside contributor can influence as a code-injection sink.

Execution Workflow

Follow these steps in order for every workflow reviewed.

Step 1 — Map the Triggers and Trust Level

Read every on: trigger and classify the workflow's privilege: