Guide · Agent skill
How to use the Cross-Origin Security Review agent skill
Updated · written with AI from the project's own documentation and checked against our catalogue data
The Cross-Origin Security Review skill (cross-origin-security) is an agent skill that tells an AI coding agent to review your web code for cross-origin security mistakes. It is aimed at front-end and full-stack developers who want a checklist-style pass over scripts, client components and bundles before release. After reading this guide you will know what the skill checks, how to install it, how to ask for a review in plain language, and what its limits are.
Cross-Origin Security Reviewthedaviddias/Front-End-Checklist · MIT Download the skill Quick start
- 1Download the ZIP. Download the cross-origin-security skill ZIP from this page.
- 2Unzip the skill. Unzip the file so you have a cross-origin-security folder containing SKILL.md and references/rule.md.
- 3Place it in a skills folder. Move the folder into ~/.claude/skills/ for personal use, or into .claude/skills/ inside a project.
- 4Or upload it in the Claude apps. In the Claude apps, go to Settings, then Capabilities, then Skills, and upload the skill.
- 5Ask for a review. Describe the code or flow to check in plain language, such as a login callback or a postMessage listener.
- 6Verify the fixes. Apply the suggested changes and confirm them in the browser, as the skill recommends.
What is an agent skill and what does this one make the agent do?
An agent skill is a folder containing a SKILL.md file with instructions that an AI agent loads when a task matches the skill's description. This skill's description tells the agent to use it when reviewing scripts, client components, bundles or runtime behavior related to handling cross-origin requests securely.
Once active, the agent looks for postMessage listeners that do not check event.origin, overly permissive CORS settings on authenticated routes, missing CSRF protection, and redirect parameters that accept arbitrary external URLs. It then suggests fixes such as origin allow-lists. It can also explain how the Same-Origin Policy, CORS and postMessage work.
- Flags postMessage listeners that process data without checking event.origin
- Flags Access-Control-Allow-Origin: * on authenticated endpoints
- Flags missing CSRF protections and redirect targets such as next, redirect or callbackUrl
- Checks login, logout, SSO and return-to flows for open redirects
How do you install the cross-origin-security skill?
Download the ZIP for the skill from this page and unzip it. For a personal setup, place the unzipped skill folder in ~/.claude/skills/. For a single project, place it in the project's .claude/skills/ folder instead.
You can also upload the skill in the Claude apps. Go to Settings, then Capabilities, then Skills, and upload the skill there.
- Personal: ~/.claude/skills/
- Project only: .claude/skills/ inside your project
- Claude apps: Settings → Capabilities → Skills
How do you trigger the skill with a natural request?
You do not need a special command. Describe the task in plain language and mention the code you want reviewed. The skill's description matches requests about reviewing scripts, client components, bundles or runtime behavior for cross-origin handling, so wording like the examples below should load it.
Point the agent at the actual files. The skill's own guidance asks for exact imports, event handlers and runtime side effects to be flagged, so naming the component or file helps.
- "Review my login callback page for open redirects."
- "Check the postMessage listeners in src/widgets for missing origin checks."
- "Audit our CORS config for the /api/account routes before release."
What does a review look like in practice?
The skill's Code Review guidance asks the agent to flag exact imports, event handlers, runtime side effects or blocking operations that break the rule, and to state how the change should be verified in the browser. Expect the agent to show you the risky code, explain why it is risky, and propose a change.
A typical fix from the reference material validates the origin before touching the data. The reference compares event.origin against an allow-list and rejects anything else with a console warning, then parses the message only after that check passes.
- Expect the flagged code, the reason it is unsafe and a suggested fix
- Expect a note on how to confirm the fix in the browser
- Redirect fixes usually reject or allow-list the destination before navigating
const ALLOWED_ORIGINS = ['https://app.example.com', 'https://admin.example.com']
window.addEventListener('message', (event) => {
if (!ALLOWED_ORIGINS.includes(event.origin)) {
console.warn('Rejected message from:', event.origin)
return
}
// parse and handle event.data only after the origin check
})What does the skill need, and is it safe to run?
The skill lists no requirements. It does not run scripts and does not use the network, so it works only by instructing the agent using the files it includes. The skill includes SKILL.md and references/rule.md, and the reference file holds the fuller examples.
Because it runs no scripts, the main safety question is the one you already face with any code review: the agent reads your code, so check its suggested changes before you apply them.
- Requires: nothing listed
- Runs scripts: no
- Uses network: no
- Files: SKILL.md and references/rule.md
What are the limits and useful tips?
The skill is a checklist-style prompt. It may miss issues or suggest fixes that are incomplete, so it is not a replacement for a professional security audit. It also needs code to look at: output is code-level advice, so it is of little use without a codebase.
Get the most from it by reviewing one flow at a time, such as login and logout, or a single widget, and by testing each suggested fix in the browser as the skill recommends.
- Review one flow or component per request for focused results
- Test every fix in the browser before shipping
- Treat the output as a checklist, not an audit
What people use it for
- Reviewing postMessage listeners in an embedded iframe or third-party widget before release
- Checking login, logout and SSO return-to parameters for open redirects
- Auditing CORS settings on routes that serve logged-in users
- Learning why event.origin checks and allow-lists are needed
Frequently asked questions
How do I install the cross-origin-security skill?
Download the ZIP from this page and unzip it. Put the folder in ~/.claude/skills/ for personal use or in a project's .claude/skills/ folder, or upload it in the Claude apps under Settings, Capabilities, then Skills.
What do I need to use this skill?
The skill lists no requirements. It does not run scripts and does not use the network. You do need a codebase or code snippets for the agent to review.
Is the cross-origin-security skill free?
It is released under the MIT licence. The source does not mention pricing, so check the licence text for the terms that apply.
Can this skill replace a security audit?
No. The skill is a checklist-style prompt and may miss issues or suggest incomplete fixes. Use it as a first pass and bring in a professional audit for production systems.
Does the skill check CSRF and CORS?
It flags overly permissive CORS settings on authenticated routes and missing CSRF protections. It suggests reviewing CORS configuration and using SameSite cookies, but it is not a full test of either.
Who is this skill not for?
Non-technical users without a codebase to review. The output is code-level advice, so it needs real code to be useful.
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Our verdict
This skill suits front-end and full-stack developers who want a focused checklist pass over postMessage, CORS, CSRF and redirect handling before release, and developers learning why these checks matter. It is not a substitute for a professional security audit, and it is of little use to anyone without code to review.
Download the skill