Github Actions Security

GitHub Actions Security Skill

A practical skill for auditing and hardening GitHub Actions workflows against supply chain attacks, secret theft, poisoned pipeline execution, and excessive token permissions.

Source reference

Based on: https://corgea.com/learn/github-actions-security-checklist
Also informed by: GitHub's 2026 Actions security roadmap, OpenSSF, OWASP CI/CD Top 10, and GitHub Security Lab guidance on preventing pwn requests.

When the user shares a workflow file

1. Read the YAML carefully.
2. Check it against all 10 checklist areas below.
3. Report findings grouped by risk area with severity (Critical / High / Medium / Low).
4. Provide concrete fixed snippets for every finding.
5. Summarize with a prioritized remediation list.

The five controls to always check first

Before anything else, verify these five — they cover the highest-impact failures:

Full checklist (10 areas)

1. Organization and repository defaults

• Workflow token permissions set to read-only by default
• "Allow GitHub Actions to create and approve pull requests" is disabled
• Fork PR workflow approval set to "Require approval for all outside collaborators" in Settings → Actions → Fork pull request workflows
• Allowed actions restricted to GitHub-owned, verified creators, or an explicit allowlist
• .github/workflows/ covered by CODEOWNERS
• Branch protection / repository rulesets require PR review, block force pushes, require status checks, dismiss stale approvals

2. Explicit workflow permissions

• permissions: declared at workflow or job level (not relying on defaults)
• Write scopes granted only to jobs that need them
• id-token: write granted only to OIDC jobs, not globally
• Read-only validation separated from privileged publishing jobs

Minimal safe pattern:

permissions: {}         # deny all at workflow level

jobs:
  build:
    permissions:
      contents: read    # grant only what this job needs
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@<full-commit-sha>

Trusted-build / privileged-publish split pattern:

Run untrusted code in a pull_request workflow (no secrets, read-only), then publish only after CI passes via a workflow_run workflow that holds credentials:

# workflow-ci.yml — triggered by pull_request, no secrets, read-only
on: [pull_request]
jobs:
  test:
    permissions:
      contents: read
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
      - run: npm test

# workflow-publish.yml — triggered only after CI passes on main
on:
  workflow_run:
    workflows: ["CI"]
    types: [completed]
    branches: [main]
jobs:
  publish:
    if: github.event.workflow_run.conclusion == 'success'
    permissions:
      contents: write
      id-token: write
    environment: production
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
      - run: npm publish

3. Dangerous triggers and untrusted execution paths

• pull_request_target avoided in public repos; if used, never checks out or executes PR head code without maintainer review
• workflow_run workflows check the upstream event before privileged actions
• issue_comment, pull_request_review, pull_request_review_comment triggers audited for privilege
• pull_request used for untrusted fork code (withholds secrets and write permissions by default)
• Stale PRs closed or rebased after fixing a vulnerable workflow

workflow_run guard pattern: A workflow_run job runs with the base repo's full permissions even when the upstream workflow was triggered by a fork PR. Always check the upstream trigger and repo before taking any privileged action:

# ❌ Dangerous: privileged job runs regardless of what triggered upstream
on:
  workflow_run:
    workflows: ["CI"]
    types: [completed]

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - run: ./deploy.sh  # runs even for fork PRs!

# ✅ Guard with explicit checks before privileged steps
jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - name: Abort if triggered by a fork PR
        if: |
          github.event.workflow_run.event == 'pull_request' &&
          github.event.workflow_run.head_repository.full_name != github.repository
        run: |
          echo "Refusing to run privileged job for fork PR"
          exit 1
      - run: ./deploy.sh  # only reached for internal PRs or push events

4. Script injection prevention

Untrusted values include: branch names, PR titles/bodies, issue titles/bodies, labels, comments, commit messages, artifact contents from untrusted workflows.

Never interpolate untrusted context directly into run: blocks:

# ❌ Unsafe
- run: echo "Branch is ${{ github.head_ref }}"

# ✅ Safe — pass through environment variable
- run: echo "Branch is $BRANCH"
  env:
    BRANCH: ${{ github.head_ref }}

Checklist:

• Untrusted context passed through environment variables, not inline ${{ }}
• Shell variables quoted when used
• Purpose-built actions preferred over inline shell for parsing untrusted input
• No untrusted data written to GITHUB_ENV or GITHUB_PATH
• No untrusted data written to GITHUB_OUTPUT or GITHUB_STEP_SUMMARY
• Artifact contents from PR workflows treated as attacker-controlled

5. Third-party action pinning

• All third-party actions pinned to full commit SHA (not a tag or branch)
• SHA verified to belong to the original repo, not a fork
• Preference for GitHub-owned or verified creator actions
• Transitive action dependencies reviewed — a pinned top-level action can still call nested actions by mutable tag; review the action's own action.yml for uses: references in composite steps, or use zizmor/actionlint to detect transitive unpinned references
• Dependabot or Renovate configured to update SHAs through reviewed PRs
• Dependency graph and Dependabot alerts enabled for actions

Example:

# ❌ Mutable tag — can be moved by attacker
- uses: actions/checkout@v4

# ✅ Immutable SHA
- uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2

6. Secret hygiene

• Secrets scoped to environments with protection rules and required reviewers for production
• Secrets passed only to the specific step that needs them (not job-level env:)
• Secrets never passed as command-line arguments (use env vars)
• Secrets (including encoded or transformed forms) never echoed
• secrets: inherit avoided in reusable workflows; only named secrets passed
• Stale secrets rotated and removed

7. OIDC for cloud credentials

• Cloud trust policies configured with repo, branch, environment, and workflow constraints
• id-token: write granted only to the job that exchanges the token
• Cloud roles limited to the exact operation (deploy, publish, etc.)
• Environment approvals required for production OIDC jobs

AWS IAM trust policy — scoped to a specific repo and environment:

{
  "Condition": {
    "StringEquals": {
      "token.actions.githubusercontent.com:sub":
        "repo:myorg/myrepo:environment:production",
      "token.actions.githubusercontent.com:aud": "sts.amazonaws.com"
    }
  }
}

⚠️ Avoid StringLike with wildcards unless you explicitly need flexibility. repo:myorg/* trusts every repository in the org — a compromised repo becomes a path to production credentials. Prefer exact repo:org/repo:environment:name claims.

8. Runner hardening

• Self-hosted runners not used for public repositories
• Ephemeral or just-in-time runners used for sensitive private workflows (destroyed after one job)
• Runner groups separated by trust level (public / private / release / production)
• Runner network egress restricted to required endpoints only
• Runner registrations and unexpected public repo creation monitored

9. Artifacts, caches, and release workflows

• Artifact uploads scoped to explicit paths (not path: .)
• .env, keys, credentials, and generated configs excluded from artifact uploads
• persist-credentials: false set on actions/checkout unless later steps need the token
• Untrusted artifacts extracted outside the workspace to a temporary directory
• Secrets and release credentials not cached
• Caches avoided in privileged release workflows
• Provenance and attestations used for package releases where supported

Cache poisoning: Cache keys are deterministic (e.g. based on hashFiles('**/package-lock.json')), so a fork PR can compute the same key, write a poisoned cache entry first, and have it restored by your privileged release workflow. Mitigate by scoping cache keys to protected branches and never restoring caches in release jobs:

# Scope cache key to the current branch — fork PRs write to a different key
- uses: actions/cache@<sha>
  with:
    path: ~/.npm
    key: ${{ runner.os }}-${{ github.ref }}-${{ hashFiles('**/package-lock.json') }}

10. Continuous detection

• Workflow linting in CI using both complementary tools:
  • actionlint — syntax validation, shellcheck integration, expression type checking; catches malformed workflows and shell errors
  • zizmor — security-focused; catches unsafe interpolation, dangerous triggers, unpinned actions
• OpenSSF Scorecard run to surface supply chain posture across the repo
• Code scanning enabled for workflow vulnerabilities (CodeQL supports common patterns)
• Action dependency changes reviewed in PRs like application dependency updates
• Alerts on failed attempts to use blocked actions or unpinned references
• Incident playbook documented: disable workflows → revoke tokens → rotate secrets → block actions → inspect runner activity

Rollout order for hardening many repos

1. Inventory — workflows with write permissions, secrets, release jobs, self-hosted runners, public fork triggers
2. Org defaults — read-only token permissions, action restrictions, CODEOWNERS, PR approval protections
3. High-risk patterns — pull_request_target, workflow_run, untrusted interpolation, GITHUB_ENV, broad secret exposure
4. Pin actions — convert mutable references to full SHAs, add Dependabot/Renovate
5. OIDC migration — start with production deploy and package publish workflows
6. Monitoring — workflow linting, dependency alerts, audit logs, runner registration alerts

Severity classification guide

Key external references

• GitHub Actions security hardening guide
• GitHub Security Lab: preventing pwn requests
• OpenSSF: mitigating attack vectors in GitHub workflows
• OWASP: Poisoned Pipeline Execution
• GitHub 2026 Actions security roadmap