CORS

CORS Header Generator

Generate Access-Control-* headers for API and preflight responses

API & HTTP

🔒 100% client-side — your data never leaves this page

Maintained by ToolsKit Editorial Team•Updated: May 19, 2026•Reviewed: May 19, 2026

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Page mode

CORS Policy Inputs

Quick CTA

Start with Origin, methods, and headers to generate copy-ready CORS response headers immediately; preflight scenarios stay in Deep.

Allowed OriginReflect request origin dynamically (Access-Control-Allow-Origin = request Origin)Allow credentials (Cookies / Authorization)Allowed MethodsAllowed HeadersExpose HeadersPreflight Max-Age (seconds)Allow private network access (Access-Control-Allow-Private-Network)

Output

Generated CORS headers appear here

100% client-side

Next step workflow

Validate request Content-Type Inspect full header block Check cookie policy together

Page reading mode

Deep expands pitfalls, recipes, snippets, FAQ, and related tools when you need troubleshooting or deeper follow-through.

About this tool

CORS Header Generator helps backend and gateway teams build correct Access-Control-* response headers quickly. Configure allowed origin strategy, methods, request headers, exposed headers, credentials, and preflight max-age, then copy ready-to-use output for servers or reverse proxies. The generator also enforces an important rule: when credentials are enabled, wildcard origin is invalid in browsers. This tool is useful for debugging frontend-to-API calls, fixing preflight failures, and producing safe CORS policies that are strict enough for production. All logic runs client-side and your inputs never leave the browser.

Compare & Decision

Wildcard origin vs explicit origin

Wildcard origin

Use it only for truly public, non-credentialed responses.

Explicit origin

Use it for authenticated or tenant-specific browser requests.

Note: If cookies or auth headers are involved, explicit origin is the safe path.

Wildcard CORS policy vs allowlist-origin CORS policy

Wildcard policy

Use only for fully public non-credentialed endpoints.

Allowlist policy

Use for authenticated APIs and controlled integrations.

Note: Allowlist policies reduce accidental cross-origin data exposure.

Static CORS headers vs request-aware dynamic CORS

Static headers

Use for small predictable integrations.

Dynamic origin evaluation

Use for multi-tenant or partner-specific origin rules.

Note: Dynamic evaluation should still be explicit and auditable.

Fast pass vs controlled workflow

Fast pass

Use for low-impact exploration and quick local checks.

Controlled workflow

Use for production delivery, audit trails, or cross-team handoff.

Note: Cors Header Generator is more reliable when acceptance criteria are explicit before release.

Direct execution vs staged validation

Direct execution

Use for disposable experiments and temporary diagnostics.

Stage + verify

Use when outputs will be reused by downstream systems.

Note: Staged validation reduces silent compatibility regressions.

Failure Input Library

Credentials allowed with wildcard origin

Bad input: `Access-Control-Allow-Origin: *` with `Allow-Credentials: true`.

Failure: Browsers reject responses and integration appears flaky.

Fix: Return explicit origin when credentials are enabled.

Preflight missing required methods/headers

Bad input: OPTIONS response omits actual request method/header allowance.

Failure: Frontend fails before request reaches backend logic.

Fix: Mirror real method/header requirements in preflight policy.

Input assumptions are not normalized

Bad input: Production-safe defaults are not enforced.

Failure: Output appears valid locally but fails during downstream consumption.

Fix: Normalize contracts and enforce preflight checks before export.

Compatibility boundaries are implicit

Bad input: Output-shape changes are not versioned for consumers.

Failure: Same source data yields inconsistent outcomes across environments.

Fix: Declare compatibility constraints and verify with an independent consumer.

Direct Answers

Q01

Why does wildcard origin break when credentials are enabled?

Browsers reject credentialed CORS responses when Access-Control-Allow-Origin is `*`, so you must return a specific origin.

Q02

When should I add Vary: Origin?

Add it when you dynamically reflect origins or return different CORS headers per request, so shared caches do not reuse the wrong response.

Quick Decision Matrix

Public read-only assets without cookies

Recommend: Use minimal wildcard policy with strict method scope.

Avoid: Avoid enabling credentials unnecessarily.

Authenticated APIs accessed by known web apps

Recommend: Use explicit allowlist origins and tested preflight behavior.

Avoid: Avoid wildcard + credentials combinations.

Local exploration and temporary diagnostics

Recommend: Use fast pass with lightweight verification.

Avoid: Avoid promoting exploratory output directly to production artifacts.

Production release, compliance, or cross-team handoff

Recommend: Use staged workflow with explicit validation records.

Avoid: Avoid one-step execution without replayable evidence.

Failure Clinic (Common Pitfalls)

Using `*` with Access-Control-Allow-Credentials: true

Cause: That combination is invalid in browsers and leads to blocked credentialed requests.

Fix: Return the exact requesting origin and add `Vary: Origin` when the value is dynamic.

Preflight misses required methods or headers

Cause: The browser asks for a method or custom header that your response does not explicitly allow.

Fix: Mirror the real request shape in the generator and include every allowed method and header name.

Scenario Recipes

Fix a credentialed browser request

Goal: Generate a CORS response that works for cookies or auth headers without triggering browser rejection.

Set a specific allowed origin instead of wildcard.
Enable credentials only when the request really needs them.
Copy the generated headers and verify the response with a real preflight request.

Result: You get a stricter CORS policy that still works for authenticated browser calls.

Cors Header Generator readiness pass for migration cutover guardrails

Goal: Validate assumptions before output enters shared workflows.

Run representative samples and capture output structure.
Replay edge cases with downstream acceptance criteria.
Publish only after sample and edge-case checks both pass.

Result: Delivery quality improves with less rollback and rework.

Cors Header Generator incident replay for multi-environment consistency verification

Goal: Convert recurring failures into repeatable diagnostics.

Rebuild problematic inputs in an isolated environment.
Compare expected and actual outputs against explicit pass criteria.
Document reusable runbook steps for on-call and handoff.

Result: Recovery time drops and operational variance shrinks.

Production Snippets

Credentialed CORS response baseline

HTTP

Access-Control-Allow-Origin: https://app.example.com
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Authorization, Content-Type
Vary: Origin

Suggested Workflow

CORS Credentials + Wildcard Origin Error: Practical Fix Guide

Fix the classic browser CORS block when credentials are enabled with Access-Control-Allow-Origin=*.

SameSite=None Requires Secure: Cookie Fix Playbook

Resolve cross-site login/session failures caused by cookie attribute mismatch in modern browsers.

Cache-Control no-store + max-age Conflict: How to Fix Caching Chaos

Diagnose and resolve contradictory cache directives that cause random hit/miss behavior.

Use It In Practice

CORS Header Generator is most reliable with real inputs and scenario-driven decisions, especially around "Public read-only assets without cookies".

Use Cases

When Public read-only assets without cookies, prioritize Use minimal wildcard policy with strict method scope..
When Authenticated APIs accessed by known web apps, prioritize Use explicit allowlist origins and tested preflight behavior..
Compare Wildcard origin vs Explicit origin for Wildcard origin vs explicit origin before implementation.

Quick Steps

Set a specific allowed origin instead of wildcard.
Enable credentials only when the request really needs them.
Copy the generated headers and verify the response with a real preflight request.

Avoid Common Mistakes

Common failure: Browsers reject responses and integration appears flaky.
Common failure: Frontend fails before request reaches backend logic.

Frequently Asked Questions

What headers can this CORS generator produce?

It generates Access-Control-Allow-Origin, Methods, Headers, Expose-Headers, Allow-Credentials, Max-Age, and optional private-network header.

Why can't I use wildcard origin with credentials?

Browsers block credentialed CORS responses when Access-Control-Allow-Origin is *, so you must return a specific origin value.

When should I include Vary: Origin?

Include it when origin is dynamic or not wildcard so caches distinguish responses by request origin.

What is Access-Control-Max-Age used for?

It controls how long preflight results can be cached, reducing repeated OPTIONS requests.

Can this help fix preflight failures?

Yes. You can align allowed methods and headers with browser preflight requirements and copy corrected response headers.

Does this tool send configuration to a server?

No. Header generation is fully client-side.