nous-hermeshub 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.
When to Use
- You are reviewing GitHub Actions workflows for exploitable security issues.
- The task requires tracing a concrete attack path from an external attacker to workflow execution or secret exposure.
- You need a security review of workflow files, composite actions, or workflow-related scripts with evidence-based findings only.
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 definitionsaction.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_dispatchinput injection — requires write access to trigger- Expression injection in
push-only workflows on protected branches workflow_callinput 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:
- Entry point — How does the attacker get in? (fork PR, issue comment, branch name, etc.)
- Payload — What does the attacker send? (actual code/YAML/input)
- Execution mechanism — How does the payload run? (expression expansion, checkout + script, etc.)
- Impact — What does the attacker gain? (token theft, code execution, repo write access)
- 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/checkoutwithref: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 everyrun: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, notif:,with:, or job-levelenv:
Check 3: Unauthorized Command Execution
Does an issue_comment-triggered workflow execute commands without authorization?
- Is there an
author_associationcheck? - 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:
- Read the full workflow — don't rely on grep output alone
- Trace the trigger — confirm the event and check
if:conditions that gate execution - Trace the expression/checkout — confirm it's in a
run:block or actually references fork code - Confirm attacker control — verify the value maps to something an external attacker sets
- 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
## 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."
Limitations
- Use this skill only when the task clearly matches the scope described above.
- Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
- Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.