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Security48k · #3251

hunt-cors

Makes your agent look for incorrect CORS configurations. This prevents unauthorized access to web resources.

What it does

This tool helps find security flaws in how websites handle requests from different origins. It checks if a site improperly allows other sites to access sensitive information.

When to use it

  • ✓When you have permission to test a website's security.
  • ✓To check if a site is properly protecting user data from unauthorized access.
  • ✓To understand how website security settings (CORS) work and where they might fail.

Example of what to ask your AI

“I want to check the CORS security configuration on example.com. I have written permission to do so.”

Want more? Open the sections below.

How to use

The easiest way: copy the message below and paste it into Claude, ChatGPT, Cursor or any AI. It reads the skill and follows the instructions.

Skill link

https://github.com/sickn33/agentic-awesome-skills/blob/main/plugins/agentic-awesome-skills-claude/skills/hunt-cors/SKILL.md

Ready-to-paste message

Use this skill to help me: https://github.com/sickn33/agentic-awesome-skills/blob/main/plugins/agentic-awesome-skills-claude/skills/hunt-cors/SKILL.md Read the SKILL.md file at that link and follow its instructions for my task.
How it works

The tool sends requests to the website with different 'Origin' headers to see how the site responds. It looks for responses that indicate the site is allowing improper access to sensitive data.

Installation

Easiest way: copy the skill link and paste it into Claude, ChatGPT or Cursor asking it to follow the instructions.

In the terminal (Claude Code, Codex, Cursor), in your project folder:

npx skills add sickn33/agentic-awesome-skills --skill hunt-cors

Or download just the file:

curl -L https://raw.githubusercontent.com/sickn33/agentic-awesome-skills/main/plugins/agentic-awesome-skills-claude/skills/hunt-cors/SKILL.md -o SKILL.md
Technical content (original SKILL.md)

Original text by the author, straight from GitHub.

⚠️ AUTHORIZED USE ONLY This skill is for educational purposes or authorized security assessments only. You must have explicit, written permission from the system owner before using this tool. Misuse of this tool is illegal and strictly prohibited.

Mandatory confirmation gate Before running any command that probes, exploits, changes, persists on, extracts data from, or attempts credential access against a target:

  1. Ask the user to state the exact target URL, IP, account, or resource.
  2. Ask the user to confirm written authorization and the permitted scope.
  3. Show the exact command(s) and explain their expected effect.
  4. Wait for explicit confirmation in the current conversation.

Without that confirmation, remain read-only and provide defensive guidance only. Prefer a sandbox, disposable VM, or controlled lab.

HUNT-CORS — Cross-Origin Resource Sharing Misconfiguration

What actually pays (and what does not)

CORS pays High only when an attacker-controlled origin can perform a credentialed cross-origin read of sensitive authenticated data, and you have a browser PoC proving the response body is readable from evil.com.

Two hard browser rules that kill most "findings" — check these FIRST:

  • Access-Control-Allow-Origin: * CANNOT be combined with credentials. If the server returns ACAO: *, the browser refuses to send/expose the response for a credentials: include request. A wildcard-only endpoint is not credential-exploitable. It is only interesting if the data it serves is sensitive without a session (rare) — usually this is Informational/Low.
  • Access-Control-Allow-Credentials: true is meaningless on its own. It matters only if ACAO reflects/allows your specific attacker origin AND a cross-origin credentialed fetch actually returns a readable body. ACAC on a response that does not reflect your origin proves nothing.

If you cannot demonstrate a readable cross-origin authed body in a real browser, you do not have a High. Do not submit header-diffing alone.


Crown Jewel Targets

  • Reflect-any-origin + credentials — server echoes the Origin header AND sets ACAC: true → any site reads authed API responses. The classic High.
  • Null-origin trust — ACAO: null + ACAC: true. A sandbox iframe (or a data:/redirect chain) emits Origin: null, so any page can read authed data.
  • Subdomain-regex bypass — trusted-origin regex with a parsing flaw. The correct payload depends on which flaw (see Phase 3 — this is where most skills get it wrong).
  • Subdomain takeover → trusted origin — a dangling subdomain that the CORS policy trusts; take it over, host the PoC there (see hunt-subdomain).
  • postMessage missing/loose origin check — handler that processes event.data without strictly validating event.origin.

Attack Surface Signals

Any endpoint returning an Access-Control-Allow-Origin header
API endpoints:   /api/*, /v1/*, /graphql
Profile/account: /api/me, /api/profile, /api/user, /api/session
Secrets/tokens:  /api/tokens, /api/keys, /api/csrf, /api/account/settings
Financial:       /api/balance, /api/transactions
Admin/internal:  /api/admin/*, /api/internal/*

Prioritize endpoints that (a) require a session cookie and (b) return PII, tokens, CSRF tokens, or other secrets in the body.


Step-by-Step Hunting Methodology

Phase 1 — Discover CORS endpoints

# Probe API endpoints. Use GET (not -I): some servers only emit CORS on GET,
# and -I sends HEAD which may be handled differently.
while read url; do
  result=$(curl -s -D - -o /dev/null "$url" \
    -H "Origin: https://evil.com" \
    -H "Cookie: $SESSION_COOKIE" | grep -i "access-control")
  [ -n "$result" ] && echo "=== $url ===" && echo "$result"
done < recon/$TARGET/api-endpoints.txt

# httpx bulk check
cat recon/$TARGET/live-hosts.txt | awk '{print $1}' | \
  httpx -H "Origin: https://evil.com" -match-string "access-control-allow-origin"

Phase 2 — Reflect-any-origin + null origin

# Does the server reflect an arbitrary Origin back?
curl -s -D - -o /dev/null https://$TARGET/api/me \
  -H "Origin: https://evil.com" \
  -H "Cookie: $SESSION_COOKIE" | grep -i "access-control"

# Vulnerable (the High case):
#   Access-Control-Allow-Origin: https://evil.com   <- reflects attacker origin
#   Access-Control-Allow-Credentials: true          <- + credentials => readable
#

…
FAQ

What is CORS?

CORS (Cross-Origin Resource Sharing) is a security mechanism that allows a website to request resources (like data or APIs) from another domain. It helps protect data, but misconfigurations can create vulnerabilities.

What is a 'CORS misconfiguration'?

It's when a website is set up incorrectly, allowing untrusted sites to access or manipulate data that should be private.

Why do I need permission to use this tool?

Testing a website's security without permission is illegal. This tool should only be used on systems for which you have explicit, written authorization.

Details

Original description by the author

Hunt CORS Misconfiguration

repo: sickn33/agentic-awesome-skills

Popularity

GitHub stars
48k
Ranking position
#3251
License
MIT
Updated
10/15/2026

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