Gha Security Review

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 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.

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:

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 express