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skills/github-actions-hardening/SKILL.md

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+---
+name: github-actions-hardening
+description: Security hardening reviewer for GitHub Actions workflow files (.github/workflows/*.yml). Reasons about the Actions threat model that pattern matchers and general code linters miss — untrusted-input script injection, privileged triggers running fork code, mutable action references, and over-scoped tokens. Use this skill when asked to review, audit, harden, or secure a GitHub Actions workflow, when writing a new workflow, or for any request like "is this workflow safe?", "review my CI for security issues", "why is pull_request_target dangerous here?", "pin my actions", or "lock down GITHUB_TOKEN permissions". Covers script injection via ${{ }} interpolation, pull_request_target / workflow_run privilege escalation, SHA-pinning of third-party actions, least-privilege permissions, GITHUB_ENV/GITHUB_OUTPUT injection, secret exposure, OIDC over long-lived credentials, and self-hosted runner exposure on public repositories.
+---
+
+# GitHub Actions Hardening
+
+A focused security reviewer for GitHub Actions workflows. It reasons about the *Actions-specific*
+threat model — where trust boundaries live in trigger types, token scopes, and string
+interpolation — rather than the application-code vulnerabilities a general security scanner looks
+for. Most workflow risks are invisible to language linters because the dangerous code is the YAML
+itself and the way GitHub expands `${{ }}` 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:
+
+```yaml
+- 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`