hunt-cors

hunt-cors is a skill for Claude Code, Codex from uphiago/recon-skills. It costs 99 tokens per session (5,495 once invoked), scanned A, original, MIT.

A security-testing guide for finding unsafe Cross-Origin Resource Sharing, or CORS, settings. CORS controls whether a website can read data from another website in a user's browser.

In plain words
What is it for?
Use it to test origin validation, credential handling, subdomain rules, preflight requests, and browser-based cross-origin reads.
Why use it?
It helps distinguish harmless header settings from flaws that let an attacker-controlled site read sensitive authenticated data.

Skill for Claude CodeCodex

Written for no agent in particular: nothing here depends on one. Also seen: positional $N argument.

Good fit Use it to test origin validation, credential handling, subdomain rules, preflight requests, and browser-based cross-origin reads.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/uphiago/recon-skills/hunt-cors
About the project

Recon Skills is a pack of security-testing skills covering reconnaissance, web applications, APIs, authentication, vulnerability validation, cloud infrastructure, and reporting. Security professionals use it for authorized assessments of systems they own or have written permission to test. The catalogue entries are individual skills from the pack.

uphiago/recon-skills · 1,254 stars · on GitHub · hiago.sh

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent
npx skills add uphiago/recon-skills --skill hunt-cors
Clone the repo
git clone --depth 1 https://github.com/uphiago/recon-skills

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for hunt-cors

README.md
[![agentmods](https://agentmods.dev/badge/skills/uphiago/recon-skills/hunt-cors/github.svg)](https://agentmods.dev/skills/uphiago/recon-skills/hunt-cors)
Your own site
<a href="https://agentmods.dev/skills/uphiago/recon-skills/hunt-cors"><img src="https://agentmods.dev/badge/skills/uphiago/recon-skills/hunt-cors/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for hunt-cors

Your own site · 80×15
<a href="https://agentmods.dev/skills/uphiago/recon-skills/hunt-cors"><img src="https://agentmods.dev/badge/skills/uphiago/recon-skills/hunt-cors.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 99 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 5,495 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. A grade says what 26 rules found in the file — not that it is safe. Third-party audits
  • NVIDIA SkillSpector warn 7 Sept 2026
SkillSpector: 6 findings, up to medium

These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →

  • medium Data Exfiltration · line 75
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 98
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 114
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 143
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 165
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 362
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
How audits are shown
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00099 $0.05495
Opus 5 $0.00049 $0.02747
Sonnet 5 $0.00020 $0.01099
Haiku 4.5 $0.00010 $0.00549

Measured 9d ago against content hash 75fa3421a31b, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

hunt-cors scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 9d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

result=$(curl --max-time 30 --connect-timeout 10 -s -D - -o /dev/null "$url" \
redteam/hunt-cors/SKILL.md · 397 lines

How it starts

The opening of the file, as written. The whole thing — 397 lines — stays where its author put it; the contents beside it link to each section on GitHub.

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 trustACAO: 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.

Read the full file on GitHub · 397 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 9d ago First seen · 397 lines · 99 tokens per session scan A 75fa3421a31b

Subscribe to this mod's changes

hunt-cors is a skill published in the GitHub repository uphiago/recon-skills (1,254 stars, last pushed 11d ago), licensed MIT. It adds 99 tokens to every session and 5,495 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.

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