gha-security-review - Claude MCP Skill

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Attack patterns and real-world examples sourced from the HackerBot Claw campaign analysis
by StepSecurity (2025): https://www.stepsecurity.io/blog/hackerbot-claw-github-actions-exploitation
-->

# GitHub Actions Security Review

Find exploitable vulnerabilities in GitHub Actions workflows. Every finding MUST include a concrete exploitation scenario — if you can't build the attack, don't report it.

This skill encodes attack patterns from real GitHub Actions exploits — not generic CI/CD theory.

## Scope

Review the workflows provided (file, diff, or repo). Research the codebase as needed to trace complete attack paths before reporting.

### Files to Review

- `.github/workflows/*.yml` — all workflow definitions
- `action.yml` / `action.yaml` — composite actions in the repo
- `.github/actions/*/action.yml` — local reusable actions
- Config files loaded by workflows: `CLAUDE.md`, `AGENTS.md`, `Makefile`, shell scripts under `.github/`

### Out of Scope

- Workflows in other repositories (only note the dependency)
- GitHub App installation permissions (note if relevant)

## Threat Model

Only report vulnerabilities exploitable by an **external attacker** — someone **without** write access to the repository. The attacker can open PRs from forks, create issues, and post comments. They cannot push to branches, trigger `workflow_dispatch`, or trigger manual workflows.

**Do not flag** vulnerabilities that require write access to exploit:
- `workflow_dispatch` input injection — requires write access to trigger
- Expression injection in `push`-only workflows on protected branches
- `workflow_call` input injection where all callers are internal
- Secrets in `workflow_dispatch`/`schedule`-only workflows

## Confidence

Report only **HIGH** and **MEDIUM** confidence findings. Do not report theoretical issues.

| Confidence | Criteria | Action |
|---|---|---|
| **HIGH** | Traced the full attack path, confirmed exploitable | Report with exploitation scenario and fix |
| **MEDIUM** | Attack path partially confirmed, uncertain link | Report as needs verification |
| **LOW** | Theoretical or mitigated elsewhere | Do not report |

For each HIGH finding, provide all five elements:

1. **Entry point** — How does the attacker get in? (fork PR, issue comment, branch name, etc.)
2. **Payload** — What does the attacker send? (actual code/YAML/input)
3. **Execution mechanism** — How does the payload run? (expression expansion, checkout + script, etc.)
4. **Impact** — What does the attacker gain? (token theft, code execution, repo write access)
5. **PoC sketch** — Concrete steps an attacker would follow

If you cannot construct all five, report as MEDIUM (needs verification).

---

## Step 1: Classify Triggers and Load References

For each workflow, identify triggers and load the appropriate reference:

| Trigger / Pattern | Load Reference |
|---|---|
| `pull_request_target` | `references/pwn-request.md` |
| `issue_comment` with command parsing | `references/comment-triggered-commands.md` |
| `${{ }}` in `run:` blocks | `references/expression-injection.md` |
| PATs / deploy keys / elevated credentials | `references/credential-escalation.md` |
| Checkout PR code + config file loading | `references/ai-prompt-injection-via-ci.md` |
| Third-party actions (especially unpinned) | `references/supply-chain.md` |
| `permissions:` block or secrets usage | `references/permissions-and-secrets.md` |
| Self-hosted runners, cache/artifact usage | `references/runner-infrastructure.md` |
| Any confirmed finding | `references/real-world-attacks.md` |

Load references selectively — only what's relevant to the triggers found.

## Step 2: Check for Vulnerability Classes

### Check 1: Pwn Request

Does the workflow use `pull_request_target` AND check out fork code?
- Look for `actions/checkout` with `ref:` pointing to PR head
- Look for local actions (`./.github/actions/`) that would come from the fork
- Check if any `run:` step executes code from the checked-out PR

### Check 2: Expression Injection

Are `${{ }}` expressions used inside `run:` blocks in externally-triggerable workflows?
- Map every `${{ }}` expression in every `run:` step
- Confirm the value is attacker-controlled (PR title, branch name, comment body — not numeric IDs, SHAs, or repository names)
- Confirm the expression is in a `run:` block, not `if:`, `with:`, or job-level `env:`

### Check 3: Unauthorized Command Execution

Does an `issue_comment`-triggered workflow execute commands without authorization?
- Is there an `author_association` check?
- Can any GitHub user trigger the command?
- Does the command handler also use injectable expressions?

### Check 4: Credential Escalation

Are elevated credentials (PATs, deploy keys) accessible to untrusted code?
- What's the blast radius of each secret?
- Could a compromised workflow steal long-lived tokens?

### Check 5: Config File Poisoning

Does the workflow load configuration from PR-supplied files?
- AI agent instructions: `CLAUDE.md`, `AGENTS.md`, `.cursorrules`
- Build configuration: `Makefile`, shell scripts

### Check 6: Supply Chain

Are third-party actions securely pinned?

### Check 7: Permissions and Secrets

Are workflow permissions minimal? Are secrets properly scoped?

### Check 8: Runner Infrastructure

Are self-hosted runners, caches, or artifacts used securely?

## Safe Patterns (Do Not Flag)

Before reporting, check if the pattern is actually safe:

| Pattern | Why Safe |
|---|---|
| `pull_request_target` WITHOUT checkout of fork code | Never executes attacker code |
| `${{ github.event.pull_request.number }}` in `run:` | Numeric only — not injectable |
| `${{ github.repository }}` / `github.repository_owner` | Repo owner controls this |
| `${{ secrets.* }}` | Not an expression injection vector |
| `${{ }}` in `if:` conditions | Evaluated by Actions runtime, not shell |
| `${{ }}` in `with:` inputs | Passed as string parameters, not shell-evaluated |
| Actions pinned to full SHA | Immutable reference |
| `pull_request` trigger (not `_target`) | Runs in fork context with read-only token |
| Any expression in `workflow_dispatch`/`schedule`/`push` to protected branches | Requires write access — outside threat model |

**Key distinction:** `${{ }}` is dangerous in `run:` blocks (shell expansion) but safe in `if:`, `with:`, and `env:` at the job/step level (Actions runtime evaluation).

## Step 3: Validate Before Reporting

Before including any finding, read the actual workflow YAML and trace the complete attack path:

1. **Read the full workflow** — don't rely on grep output alone
2. **Trace the trigger** — confirm the event and check `if:` conditions that gate execution
3. **Trace the expression/checkout** — confirm it's in a `run:` block or actually references fork code
4. **Confirm attacker control** — verify the value maps to something an external attacker sets
5. **Check existing mitigations** — env var wrapping, author_association checks, restricted permissions, SHA pinning

If any link is broken, mark MEDIUM (needs verification) or drop the finding.

**If no checks produced a finding, report zero findings. Do not invent issues.**

## Step 4: Report Findings

````markdown
## GitHub Actions Security Review

### Findings

#### [GHA-001] [Title] (Severity: Critical/High/Medium)
- **Workflow**: `.github/workflows/release.yml:15`
- **Trigger**: `pull_request_target`
- **Confidence**: HIGH — confirmed through attack path tracing
- **Exploitation Scenario**:
  1. [Step-by-step attack]
- **Impact**: [What attacker gains]
- **Fix**: [Code that fixes the issue]

### Needs Verification
[MEDIUM confidence items with explanation of what to verify]

### Reviewed and Cleared
[Workflows reviewed and confirmed safe]
````

If no findings: "No exploitable vulnerabilities identified. All workflows reviewed and cleared."