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CORS Misconfiguration Checker

Sends cross-origin requests with varied Origin headers to detect dangerous CORS policies that let attacker sites read authenticated responses.

HEADER: Access-Control-Allow-Origin
RISK PATTERN: Origin reflection + credentials
SPEC: WHATWG Fetch CORS protocol

What is CORS Misconfiguration Checker?

Cross-Origin Resource Sharing headers tell a browser which other websites are allowed to read a server response. A misconfigured policy — most commonly reflecting whatever Origin was sent while also allowing credentials — effectively trusts every website on the internet.

How it Works

The tool sends requests to the target URL with several test Origin header values and inspects the Access-Control-Allow-Origin and Access-Control-Allow-Credentials headers returned, flagging reflection, wildcard misuse, and weak substring-based origin matching.

Key Use Cases

Auditing an API before launch to confirm only intended frontends can read authenticated responses.

Investigating whether a session-cookie API can be read cross-origin by an arbitrary attacker-controlled site.

Catching a reflected-origin misconfiguration left over from local development that reached production.

Advanced Details

Browsers refuse the combination of a wildcard origin and Allow-Credentials outright, so a misconfigured server usually resorts to reflecting the request Origin instead — which is the pattern this tool specifically looks for.

Cybersecurity Pros & Cons

Security Advantages
  • Directly detects the origin-reflection-plus-credentials pattern that is the most common critical CORS finding.
  • Catches weak substring origin matching that allows lookalike domains such as evil-example.com.
  • Runs in seconds against any endpoint with no authentication required.
  • Flags the dangerous null-origin acceptance that sandboxed iframes can trigger deliberately.
Security Limitations
  • CORS controls whether a response can be read, not whether a request can be sent — it says nothing about CSRF exposure.
  • A correctly restrictive policy on one endpoint does not guarantee every endpoint in the API is configured the same way.
  • The check reflects only headers returned to this tool own request; server logic that varies by request path can behave differently elsewhere.
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