Workflow Trigger Security Skill

Workflow Trigger Security

When to Use This Skill

Workflow triggers control when code executes and in what security context. Choose the wrong trigger and attackers can inject code, escalate privileges, or exfiltrate secrets from forks.

The Risk

Trigger misconfiguration is the leading cause of GitHub Actions privilege escalation. pull_request_target runs fork code in the base repository context with full secret access. External contributors can weaponize this to steal credentials, push malicious code, or compromise your infrastructure.

Implementation

See the full implementation guide in the source documentation.

Techniques

Fork Workflow Security Patterns

Pattern 1: Two-Stage Fork CI

Separate untrusted fork testing from privileged operations.

# Stage 1: Test fork code with pull_request
name: Fork Tests
on:
  pull_request:
    branches: [main]

permissions:
  contents: read

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11  # v4.1.1
      - run: npm test

# Stage 2: Post results with workflow_run
name: Post Test Results
on:
  workflow_run:
    workflows: ["Fork Tests"]
    types: [completed]

permissions:
  pull-requests: write
  contents: read

jobs:
  comment:
    runs-on: ubuntu-latest
    if: github.event.workflow_run.event == 'pull_request'
    steps:
      - uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea  # v7.0.1
        with:
          script: |
            await github.rest.issues.createComment({
              issue_number: ${{ github.event.workflow_run.pull_requests[0].number }},
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: 'Tests completed. Check workflow run for results.'
            });

Stage 1 runs untrusted code without secrets. Stage 2 runs trusted code with write permissions. The workflow_run trigger bridges the security boundary.

Pattern 2: Approval Gate for Fork Deployments

Require manual approval before deploying fork code.

name: Preview Deployment
on:
  pull_request_target:
    branches: [main]

permissions:
  contents: read
  pull-requests: write

jobs:
  deploy-preview:
    runs-on: ubuntu-latest
    environment: pr-previews
    steps:
      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11  # v4.1.1
        with:
          ref: ${{ github.event.pull_request.head.sha }}

      - uses: google-github-actions/auth@55bd3a7c6e2ae7cf1877fd1ccb9d54c0503c457c  # v2.1.2
        with:
          workload_identity_provider: ${{ secrets.WIF_PROVIDER }}
          service_account: ${{ secrets.WIF_SERVICE_ACCOUNT }}

      - run: npm run deploy-preview

Configure the pr-previews environment in Settings → Environments with required reviewers and deployment branch restrictions.

Pattern 3: Fork PR Security Validation

name: Fork Security Scan
on:

*See [reference.md](reference.md) for additional techniques and detailed examples.*


## Comparison

Understanding the difference between these triggers is critical for fork security.

### `pull_request` Trigger

**Security Context**: Runs in fork's security context

**Code Executed**: From pull request HEAD

**Secret Access**: None

**GITHUB_TOKEN Permissions**: Read-only by default

**Safe For**: Public repositories accepting fork contributions

**Use Cases**: Testing, linting, build verification, security scanning

**Example Safe Fork CI**:


*See [examples.md](examples.md) for detailed code examples.*

**Why This Is Safe**:

1. Fork code runs in isolated context
2. No access to repository secrets
3. GITHUB_TOKEN is read-only
4. Cannot push code or modify releases
5. Cannot access organization resources

### `pull_request_target` Trigger

**Security Context**: Runs in base repository context

**Code Executed**: From base branch, NOT pull request code

**Secret Access**: Full repository and organization secrets

**GITHUB_TOKEN Permissions**: Write by default

**Dangerous For**: Any workflow that checks out or executes PR code

**Valid Use Cases**: Commenting on PRs, labeling PRs, triggering external systems, publishing previews after approval

**Example DANGEROUS Pattern**:


*See [examples.md](examples.md) for detailed code examples.*

**Why This Is Dangerous**:

1. Fork code executes in base repository security context
2. Full access to all secrets
3. GITHUB_TOKEN has write permissions by default
4. Attacker can exfiltrate credentials in malicious `package.json` scripts
5. Attacker can push code, create releases, or modify workflows

**Example Safe Pattern with Approval**:


*See [examples.md](examples.md) for detailed code examples.*

**Safe Pattern Requirements**:

1. Minimal permissions (no `contents: write`)
2. Environment protection with required reviewers
3. OIDC authentication instead of stored secrets
4. Explicit checkout only after approval gate
5. Audit logging for all preview deployments

### Trigger Comparison Table

| Aspect | `pull_request` | `pull_request_target` |
| ------ | -------------- | --------------------- |
| **Execution Context** | Fork HEAD | Base branch |
| **Secret Access** | None | Full access |
| **GITHUB_TOKEN** | Read-only | Write |
| **Use Case** | Test fork PRs | Comment on PRs |
| **Risk Level** | Low | High |
| **Safe for Forks** | Yes | Only with approval |


## Examples

See [examples.md](examples.md) for code examples.


## Full Reference

See [reference.md](reference.md) for complete documentation.
## References

- [Source Documentation](https://adaptive-enforcement-lab.com/secure/github-actions-security/)
- [AEL Secure](https://adaptive-enforcement-lab.com/secure/)

Agent Skills: Workflow Trigger Security

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