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:

• push, pull_request (from same repo) → runs with the contributor's own trust
• pull_request from a fork → runs with a read-only token, no secrets (safe by design)
• pull_request_target, workflow_run, issue_comment, issues → run in the context of the base repository with a read/write token and full access to secrets, but can be triggered by outside contributors. These are the dangerous triggers.

Read references/triggers-and-privilege.md for the full trust matrix.

Step 2 — Hunt for Script Injection

For every run: block, every script: in actions/github-script, and every input to a custom action, list the ${{ }} expressions and check whether any resolve to attacker-controllable data. High-risk contexts include:

• github.event.issue.title, github.event.issue.body
• github.event.pull_request.title, github.event.pull_request.body, .head.ref, .head.label
• github.event.comment.body, github.event.review.body
• github.event.pages.*.page_name, github.event.commits.*.message, github.event.head_commit.*
• github.head_ref and any github.event.* field a fork author can set

Read references/injection.md for the complete sink list and the safe-pattern fixes.

Step 3 — Check Privileged Triggers Don't Execute Untrusted Code

If a pull_request_target or workflow_run workflow checks out PR/fork code (ref: ${{ github.event.pull_request.head.sha }}) and then runs it (build, test, install scripts, npm install with lifecycle scripts, etc.), that is remote code execution against a privileged token. Flag it as CRITICAL. The safe pattern is to split into two workflows: an unprivileged pull_request workflow that runs the untrusted code, and a privileged workflow_run workflow that only consumes its results.

Step 4 — Audit permissions:

• If there is no permissions: block, the workflow inherits the repository default, which may be read/write to everything. Flag it.
• Recommend a top-level permissions: {} (deny-all) or contents: read, then grant the minimum per job (e.g. pull-requests: write only on the job that comments).
• Flag any permissions: write-all or broad write scopes that the steps don't actually need.

Read references/permissions-and-tokens.md for the per-scope guidance and OIDC setup.

Step 5 — Audit Action References (Supply Chain)

For every uses::

• Third-party actions (not actions/* or github/*) MUST be pinned to a full 40-character commit SHA, not a tag or branch. Tags and branches are mutable; a compromised upstream action can rewrite v1 to malicious code that runs with your token and secrets.
• First-party actions/* are lower risk but SHA-pinning is still the hardened recommendation.
• Flag @main, @master, or any branch reference as HIGH — that is "latest" and can change under you at any time.
• Note the human-readable version in a trailing comment: uses: foo/bar@<sha> # v2.1.0.

Read references/supply-chain.md for pinning, Dependabot for actions, and artifact/cache risks.

Step 6 — Check Secret and Output Handling

• No secrets echoed, printed, or written to logs; no set -x / bash -x in steps that touch secrets.
• Secrets must not be passed to steps that run untrusted code or to untrusted third-party actions.
• Untrusted multiline data written to $GITHUB_ENV or $GITHUB_OUTPUT can inject environment variables or step outputs — use the random-delimiter heredoc form and never write raw user input.
• actions/checkout leaves a token on disk by default; set persist-credentials: false when the job later runs untrusted code.

Step 7 — Produce the Report

Output findings using the format in references/report-format.md: a severity summary table first, then grouped findings with file, the exact offending YAML, the risk in plain English, and a concrete before/after fix. Never auto-apply changes — present them for review.

Severity Guide

Severity	Meaning	Example
? CRITICAL	Token/secret theft or RCE reachable by an outside contributor	pull_request_target checking out and running fork code; ${{ github.event.* }} in a run: on a privileged trigger
? HIGH	Exploitable supply-chain or scope problem	Third-party action on a mutable tag/branch; write-all permissions; injection sink on issue_comment
? MEDIUM	Risk under conditions or chaining	Missing permissions: block; secret reachable by a non-fork PR author
? LOW	Hardening gap, low direct risk	First-party action not SHA-pinned; persist-credentials left default on a non-privileged job
⚪ INFO	Observation, not a vulnerability	Version comment missing next to a pinned SHA

Output Rules

• Always show a findings summary table (counts by severity) first.
• Group by issue type, not by file.
• Be exact — quote the offending line and give the line location.
• Always pair every CRITICAL/HIGH with a concrete corrected YAML snippet.
• Never claim a fork pull_request is dangerous just because it runs untrusted code — it has no secrets and a read-only token. Reserve CRITICAL for the privileged triggers.
• If the workflow is already hardened, say so and list what was checked.

Reference Files

Load these as needed:

• references/triggers-and-privilege.md — Trust matrix for every trigger, why pull_request_target and workflow_run are privileged, and the two-workflow safe pattern.
  • Search patterns: pull_request_target, workflow_run, issue_comment, fork, secrets, read-only token, trust boundary
• references/injection.md — Full list of attacker-controllable ${{ }} contexts and the env:-variable safe pattern for each sink (run, github-script, action inputs).
  • Search patterns: script injection, github.event, head_ref, issue title, env, intermediate variable, actions/github-script
• references/permissions-and-tokens.md — GITHUB_TOKEN scopes, least-privilege permissions: recipes per job type, and OIDC for cloud auth instead of long-lived secrets.
  • Search patterns: permissions, GITHUB_TOKEN, write-all, contents: read, id-token, OIDC, least privilege
• references/supply-chain.md — SHA-pinning third-party actions, Dependabot for github-actions, artifact and cache poisoning across workflow_run, and self-hosted runner exposure.
  • Search patterns: SHA pin, uses, mutable tag, Dependabot, download-artifact, cache, self-hosted runner
• references/report-format.md — Output template: summary table, finding cards, and before/after remediation blocks.
  • Search patterns: report, format, finding, summary, remediation, before, after