github-actions

# GitHub Actions CI/CD A workflow is config that runs on an event. Before you write a single step, decide three things: **which events** fire the workflow, **what permissions** the token needs, and **where credentials come from**. Get those wrong and you have a fast pipeline that leaks secrets or a secure one nobody can trigger. Everything after that — checkout, install, test, build — is just steps. This skill owns the workflow layer — the `.github/workflows/*.yml` files, their triggers, jobs, matrix, caching, secret/OIDC handling, environments and deploy gates. Route the rest out: | Not this skill | Goes to | Because | | --- | --- | --- | | The `Dockerfile`, image build strategy | `../docker/SKILL.md` | The workflow may *call* `docker build`; designing the image is not this skill. | | Branching model, PR hygiene, merge vs rebase, commit conventions | `../git-workflow/SKILL.md` | That is the source-control model, not the CI config layer. | | Release readiness checklist, changelog, the shipping decision | `../ship/SKILL.md` | Whether to release is a decision; this skill only automates the mechanics. | | Blue/green, canary, rollback *theory* | `../deployment/SKILL.md` | Actions triggers the deploy; the strategy is deployment's. | | Choosing the host and its deploy primitives | `../vercel/SKILL.md`, `../aws-essentials/SKILL.md` | Actions *triggers* the deploy; the host owns the target. | | Triaging SAST/CVE findings, threat modeling | `../secure-coding/SKILL.md` | This skill runs a scanner *as a job*; it does not interpret the report. | ## Decide the trigger first Pick the event(s) for each job class before writing YAML — the trigger decides what context and secrets the run gets. | Event | Use it for | Why | | --- | --- | --- | | `pull_request` | lint, test, build-check | Runs on the merge ref; from forks it gets **no secrets** (safe). | | `push` (to `main`) | deploy, publish artifacts, build the release | The trusted ref with full secrets/OIDC. | | `workflow_dispatch` | manual ops, one-off backfills, manual deploys | Human-triggered with inputs; auditable. | | `schedule` (cron) | nightly builds, dependency audits, cache warmers | Cron in UTC; no human in the loop. | | `release` / `push` tags | publish to a registry, cut a GitHub Release | Fires on the tag, not every commit. | | `workflow_call` | reusable workflow invoked by others | Library of jobs; never runs on its own. | | `pull_request_target` | label/comment bots that need write on forks | **Runs trusted with secrets** — never check out PR head here. | Do **not** run the same heavy job on both `push` and `pull_request` for the same commit — you pay runner minutes twice. Use `pull_request` for the checks and a separate `push: branches: [main]` job for deploy. ## Anatomy of a CI workflow The minimal good CI: scoped trigger, read-only token, concurrency that cancels stale PR runs, built-in cache. ```yaml name: CI on: push: branches: [main] pull_request: permissions: contents: read # least privilege; widen per-job only when needed concurrency: group: ci-${{ github.ref }} # one run per branch/PR cancel-in-progress: true # newer push kills the stale run (PR feedback) jobs: test: runs-on: ubuntu-latest steps: - uses: actions/checkout@v6 # first-party, current major - uses: actions/setup-node@v6 with: node-version: 22 cache: npm # built-in lockfile-keyed cache - run: npm ci # not install — fails on a stale lockfile, reproducible - run: npm run lint - run: npm test ``` Why those three lines are not optional: - `permissions: contents: read` at the top — **default token permissions may be write**, and a leaked write token can push tags or publish packages. Widen per job and only to what that job needs (`packages: write` to publish, `id-token: write` for OIDC), never at the top. - `concurrency` + `cancel-in-progress: true` — without it every push to an open PR leaves the old run finishing and billing. One group per ref keeps at most one running + one pending. - `actions/checkout@v6`, `setup-node@v6` are the current majors (checkout v6.0.2, setup-node v6.4.0). Old majors run on Node 20, removed from runners in **September 2026**; JS actions are forced onto **Node 24** by default since June 2026. ## Caching Two mechanisms, in order of preference: 1. **Built-in `cache:` on `setup-*`** — `setup-node`, `setup-python`, `setup-go`, etc. cache the package manager's store keyed on the lockfile. Free, one line. Use it. 2. **`actions/cache@v4`** — for anything else (build output, custom tool dirs, compiled artifacts). The cache key is the whole game. A cache is **immutable once written for a key** — if your key never changes, you cache stale deps forever. ```yaml # Bad — fixed key never invalidates; you restore yesterday's broken node_modules forever - uses: actions/cache@v4 with: path: ~/.npm key: npm-cache # Good — key changes when the lockfile changes; restore-keys gives a warm partial hit - uses: actions/cache@v4 with: path: ~/.npm key: ${{ runner.os }}-npm-${{ hashFiles('**/package-lock.json') }} restore-keys: | ${{ runner.os }}-npm- ``` `restore-keys` is a prefix fallback: an exact-key miss still restores the most recent cache whose key starts with the prefix, so a one-package change does not cold-start. Monorepo keys, Docker layer caching (`type=gha`), and runner-minute cost tradeoffs live in `references/caching-and-matrix.md`. ## Matrix Run one job definition across combinations — OS x version is the common case. ```yaml jobs: test: runs-on: ${{ matrix.os }} strategy: fail-fast: false # see all combos' results, not just the first failure max-parallel: 4 matrix: os: [ubuntu-latest, macos-latest] node: [20, 22, 24] exclude: - os: macos-latest # don't pay the macOS multiplier on every version node: 20 include: - os: ubuntu-latest # one extra cell: lint only on the canonical combo node: 24 lint: true steps: - uses: actions/checkout@v6 - uses: actions/setup-node@v6 with: { node-version: "${{ matrix.node }}", cache: npm } - run: npm ci && npm test ``` Set `fail-fast: false` when you want every combination's verdict (a compatibility matrix); leave it `true` (default) when one failure should abort the rest to save minutes. macOS and Windows runners bill at a multiple of Linux minutes — `exclude` the cells you do not need. ## Secrets and OIDC — the security heart The rule: **no long-lived cloud keys in repo secrets.** Use OIDC. GitHub mints a short-lived JWT per run; AWS/Azure/GCP exchange it for a token scoped to that job, valid for minutes. Nothing static to steal — by 2026, static CI credentials are a compliance violation in regulated orgs. ```yaml # Bad — static AWS keys live in the repo forever; one leak = standing access - uses: aws-actions/configure-aws-credentials@v4 with: aws-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }} aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }} # Good — OIDC: no stored keys, the role is assumed for this run only permissions: id-token: write # required for GitHub to mint the OIDC JWT contents: read steps: - uses: aws-actions/configure-aws-credentials@<full-40-char-sha> with: role-to-assume: arn:aws:iam::123456789012:role/gh-deploy aws-region: eu-west-1 ``` Hard rules: - **Never `echo` a secret** or pass it to an untrusted step. Secrets are masked in logs, but a third-party action or a crafted `printf` can exfiltrate them. - **Scope the cloud trust to repo + ref (+ environment).** The common 2026 misconfig is a trust policy with `repo:ORG/*` — that lets *any* repo in the org assume your prod role. Scope `sub` to `repo:ORG/REPO:ref:refs/heads/main` or `environment:production`. - **Gate prod with an `environment` + required reviewers** so a human approves before the deploy job runs. Per-cloud trust setup (AWS role, GCP Workload Identity Federation, Azure federated credentials), the over-permissioned-trust footgun, and a full deploy-on-tag workflow with approval are in `references/oidc-deploys.md`. ## Supply chain and least privilege - **SHA-pin third-party actions to a full 40-char commit SHA, not a tag.** Tags are mutable: the **tj-actions/changed-files compromise (2025)** retargeted *all* tags to malicious code that dumped secrets. A SHA is the only immutable reference. GitHub now offers repo/org/enterprise policy to *enforce* full-SHA pinning across the whole tree. ```yaml # Bad — mutable tag; whoever controls the repo can repoint v1 at anything - uses: some-org/some-action@v1 # Good — immutable, with a comment recording the human-readable version - uses: some-org/some-action@a1b2c3d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0 # v1.4.2 ``` First-party `actions/*` and `github/*` may stay on a major tag (GitHub controls them), but pinning everything is the stronger posture. - **`pull_request_target` + checking out the PR head = remote code execution with your secrets.** That trigger runs in the *base* repo's trusted context. If you then `checkout` `github.event.pull_request.head.sha`, you execute a fork's code with full secret access. Never combine them. ## Reuse: workflow vs composite action Both kill copy-paste; pick by scope. | You need to reuse... | Use | Note | | --- | --- | --- | | whole jobs with their own `runs-on` / `services` / matrix | reusable workflow (`on: workflow_call`) | `secrets: inherit` to forward; set `concurrency` *inside* it. | | a set of steps that run inside one existing job | composite action | Lives at `.github/actions/<name>/action.yml`. | ```yaml # caller — reuse a whole job jobs: test: uses: ./.github/workflows/reusable-test.yml secrets: inherit ``` Gotcha: `concurrency` on the job that *calls* a reusable workflow does not behave as you expect — declare it inside the called workflow. ## Deploy job pattern A deploy depends on the build, gets its own environment gate, and must **never** be cancelled mid-release. ```yaml deploy: needs: build # only deploy a green build runs-on: ubuntu-latest environment: production # required-reviewer gate lives on the environment concurrency: group: deploy-production cancel-in-progress: false # NEVER interrupt a release permissions: id-token: write contents: read steps: - uses: actions/checkout@v6 - uses: aws-actions/configure-aws-credentials@<full-40-char-sha> with: role-to-assume: arn:aws:iam::123456789012:role/gh-deploy aws-region: eu-west-1 - run: ./scripts/deploy.sh ``` `cancel-in-progress: false` here is the opposite of the CI default: cancelling a half-finished deploy can leave prod in a broken state. ## Anti-patterns | Anti-pattern | Why it bites | Do instead | | --- | --- | --- | | Third-party action pinned to a tag (`@v1`) | tj-actions 2025: tags got repointed to secret-stealing code | Pin to a full 40-char SHA, comment the version | | `permissions: write-all` or no `permissions:` block | Default token may be write; a leak can push/publish | Top-level `contents: read`, widen per job | | Static cloud keys in repo secrets | Standing credentials; one leak = lasting access | OIDC `id-token: write` + `role-to-assume` | | OIDC trust scoped to `repo:ORG/*` | Any org repo can assume your prod role | Scope `sub` to repo + ref + environment | | No `concurrency` block | PR runs pile up and bill; deploys race | `cancel-in-progress: true` for CI, `false` for deploy | | Cache key with no lockfile hash | Restores stale deps forever (immutable per key) | `key: ...-${{ hashFiles('**/lock') }}` + restore-keys | | `pull_request_target` + checkout PR head | Runs fork code with your secrets (RCE) | Use `pull_request`; never check out untrusted head with secrets | | Same heavy job on `push` **and** `pull_request` | Double-bills runner minutes per commit | `pull_request` for checks, `push: [main]` for deploy | | `echo`-ing a secret to debug | Crafted steps/actions exfiltrate the masked value | Never print secrets; use OIDC short-lived tokens | ## Verify After writing or editing workflows, run the static check on the repo: ```bash skills/github-actions/scripts/verify.sh . ``` It globs `.github/workflows/*.{yml,yaml}`, runs `actionlint` if present, and independently flags unpinned third-party actions, missing `permissions:`, an OIDC nudge for jobs using cloud secrets, and the `pull_request_target` + PR-head footgun. It exits non-zero only on a hard error, so it works as a CI gate.