Cross-Origin Resource Sharing (CORS)
An HTTP-header-based security protocol that instructs web browsers to allow front-end JavaScript running at one origin to access restricted resources on a server hosted at a different origin.
Entity Relationships (GEO Conceptual Hierarchy)
Taxonomy ModelTechnical Explanation & Operational Mechanics
Because web browsers enforce the Same-Origin Policy by default, client-side fetch or XMLHttpRequest calls to external domains are blocked from reading the response. CORS provides an opt-in mechanism where servers advertise which origins are allowed to read their data via specialized HTTP response headers, such as Access-Control-Allow-Origin, Access-Control-Allow-Methods, and Access-Control-Allow-Headers. For non-simple requests (such as requests using PUT, DELETE, or custom JSON headers), browsers automatically dispatch an initial HTTP OPTIONS preflight request to verify server consent before executing the actual request.
Technical Example & Practical Implementation
Browser verifies cross-origin permissions with OPTIONS before sending a JSON POST payload
// 1. Browser sends automatic preflight check:
OPTIONS /api/v1/profile HTTP/1.1
Host: api.example.com
Origin: https://app.client.com
Access-Control-Request-Method: POST
Access-Control-Request-Headers: Authorization, Content-Type
// 2. Server grants cross-origin permission:
HTTP/1.1 204 No Content
Access-Control-Allow-Origin: https://app.client.com
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Authorization, Content-Type
Access-Control-Max-Age: 86400
// 3. Browser executes the actual POST request:
POST /api/v1/profile HTTP/1.1
Host: api.example.com
Origin: https://app.client.com
Content-Type: application/jsonRelated Topic Ecosystem & Next Steps
Related Technical Articles
CSP vs CORS: Differences in Browser Defense Mechanisms
Distinguishing outbound resource loading restrictions from inbound server access control.
Troubleshooting: No Access-Control-Allow-Origin Header Present
Diagnose and fix the most common browser cross-origin blocking error with production configs.
Developer References & Specifications
Continue Learning
CORS Policy & Preflight Handshake Lab
Simulate preflight OPTIONS probes and adjust Access-Control-Allow-* headers inside the browser sandbox.