A critical vulnerability has been identified in the way several popular web frameworks handle Cross-Origin Resource Sharing (CORS) with 'Wildcard Subdomains'. When configurations like Access-Control-Allow-Origin: *.example.com are used, attackers can exploit vulnerabilities in any low-security subdomain (e.g., a marketing site) to steal sensitive session cookies or CSRF tokens from the main application. This 'Subdomain Takeover-to-CORS' chain is becoming a top threat for large enterprises. Best practice dictates using an explicit allow-list of origins rather than wildcard patterns to maintain strict origin isolation.