# cors-cross-origin-misconfiguration

Guides testing of CORS misconfigurations, origin reflection, and credentialed API access. Useful when auditing cross-origin boundaries and cache policies.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 3700 installs · ★ 2547
- Author: yaklang
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/yaklang/hack-skills/tree/HEAD/skills/cors-cross-origin-misconfiguration/SKILL.md
- Updated: 2026-09-13
- HTML: https://affpapa.org/en/nevibecode/skills/cors-cross-origin-misconfiguration

## What it does

This skill inspects cross-origin resource sharing policies for authenticated APIs. It targets insecure reflection of the `Origin` header and broken credential handling. Such errors allow unauthorized browser contexts to read sensitive user records.

The playbook covers weak allowlist regular expressions, sandbox `null` origins, and missing `Vary: Origin` headers. When proxies miss that header, caches can serve one origin's response to another user. It also highlights attack chains where subdomain cross-site scripting bypasses origin filters.

This guide focuses purely on policy analysis. Extended workflows such as JSONP hijacking and local lab setups require loading the companion `SCENARIOS.md` file.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill cors-cross-origin-misconfiguration -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/cors-cross-origin-misconfiguration/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/cors-cross-origin-misconfiguration/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill cors-cross-origin-misconfiguration -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/cors-cross-origin-misconfiguration/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill cors-cross-origin-misconfiguration -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/cors-cross-origin-misconfiguration/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install yaklang/hack-skills cors-cross-origin-misconfiguration
```

2. **Manually: put the files in place**
- File: `.github/skills/cors-cross-origin-misconfiguration/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill cors-cross-origin-misconfiguration -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/yaklang/hack-skills --path skills/cors-cross-origin-misconfiguration
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/cors-cross-origin-misconfiguration/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill cors-cross-origin-misconfiguration -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/yaklang/hack-skills/tree/HEAD/skills/cors-cross-origin-misconfiguration/SKILL.md

## When to use it
- inspect origin allowlists for regex bypasses and loose anchors
- verify whether the API accepts credentialed requests from Origin: null
- spot cache poisoning risks caused by a missing Vary: Origin header
- audit cross-origin read access across subdomains

## Example prompts
- Check our CORS configuration on /api/user for reflected origin vulnerabilities
- Analyze this origin validation regex for bypass patterns
- Explain the risk of missing Vary: Origin with dynamic ACAO headers

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 9f7a28a5b5bb6545.

BEGIN THIRD-PARTY FILE 9f7a28a5b5bb6545
```text
---
name: cors-cross-origin-misconfiguration
description: >-
  CORS misconfiguration testing playbook. Use when analyzing cross-origin trust, credentialed browser reads, origin reflection, preflight policy bugs, and browser-based access to authenticated APIs.
---

# SKILL: CORS Misconfiguration — Credentialed Origins, Reflection, and Trust Boundary Errors

> **AI LOAD INSTRUCTION**: Use this skill when browsers can access authenticated APIs cross-origin. Focus on reflected origins, credentialed requests, wildcard trust, parser mistakes, and origin allowlist bypasses. For JSONP hijacking deep dives, same-origin policy internals, honeypot de-anonymization, and CORS vs JSONP comparison, load the companion [SCENARIOS.md](./SCENARIOS.md).

### Extended Scenarios

Also load [SCENARIOS.md](./SCENARIOS.md) when you need:
- JSONP hijacking complete attack scenario — watering hole + `<script>` cross-origin data theft
- Honeypot de-anonymization via JSONP — use social platform JSONP endpoints to identify anonymous visitors
- Same-origin policy deep dive — protocol/hostname/port definition, `document.domain` subdomain relaxation and its security risks
- CORS vs JSONP technical comparison — methods, error handling, credential behavior, migration path
- CORS exploitation payloads — reflected origin with `credentials: include`, null origin via sandboxed iframe
- Dual-site attack lab pattern — localhost:8981 (target) + localhost:8982 (attacker) testing setup

## 1. WHEN TO LOAD THIS SKILL

Load when:

- Responses contain `Access-Control-Allow-Origin`, `Access-Control-Allow-Credentials`, or preflight headers
- A browser-based attack path might read authenticated API responses
- JSON endpoints appear protected from CSRF but are readable cross-origin

## 2. HIGH-VALUE MISCONFIGURATION CHECKS

| Theme | What to Check |
|---|---|
| wildcard with credentials | `Access-Control-Allow-Origin: *` plus credential support or equivalent broken behavior |
| reflected origin | server echoes arbitrary `Origin` |
| weak allowlist | suffix, prefix, substring, regex, or mixed-case matching errors |
| `null` origin | acceptance of sandboxed, file, or serialized origins |
| preflight trust | overbroad methods and headers |
| internal API exposure | admin or tenant data readable cross-origin |

## 3. QUICK TRIAGE

1. Send crafted `Origin` headers and inspect reflection.
2. Test with and without credentials.
3. Probe allowlist bypasses using attacker subdomains and parser edge cases.
4. If readable data is sensitive, chain to account or tenant impact.

## 4. RELATED ROUTES

- Session or JSON action abuse: [csrf cross site request forgery](../csrf-cross-site-request-forgery/SKILL.md)
```
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