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auditing-cors-policy

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Audit a target's CORS posture — Access-Control-Allow-Origin handling, reflected-origin bypass, credentials+wildcard mismatch, preflight OPTIONS behavior, Vary header correctness. Use when: a third-party integration is failing CORS preflight and someone proposes "just set Allow-Origin to *" as the fix, OR your bug-bounty inbox has a credential-reuse exploit chain. Threshold: any reflection of arbitrary Origin into Allow-Origin, Allow-Credentials:true with wildcard origin (browser-rejected combo but server config wrong), missing Vary:Origin on per-origin responses, preflight cached over 86400s, OR Allow-Origin trust of attacker- controlled subdomain pattern. Trigger with: "audit cors", "check cors policy", "cors bypass", "preflight check".

skills/.curated/auditing-cors-policy/SKILL.md

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Last checked
Nov 12, 2026

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Repository last updated
Nov 12, 2026
License
MIT

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# Auditing CORS Policy

## Overview

CORS misconfiguration is one of the most common middle-severity findings
in web bug bounties. The browser-enforced rules are subtle, the failure
modes are silent (the wrong cors response just works until an attacker
weaponizes it), and the "fix" engineers reach for —
`Access-Control-Allow-Origin: *` — opens the very class of attacks CORS
was meant to prevent when paired with credentials.

This skill probes each common CORS misconfiguration with synthetic
Origin headers and grades the response.
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