Summary The built-in CORS middleware ("hono/cors") parses the attacker-controlled "Access-Control-Request-Headers" request header during a preflight ("OPTIONS") request using a regular expression whose running time is quadratic in the input length. A single request carrying a long run of whitespace can consume seconds of CPU, and repeated requests can render the service unresponsive. This parsing runs under the default configuration. Details On a CORS preflight, when "allowHeaders" is not configured - the default - the middleware reflects and parses the "Access-Control-Request-Headers" value. The parser used a whitespace-tolerant regular expression whose backtracking makes the work grow quadratically (O(n²)) with the length of the value when it contains a long whitespace sequence without a delimiter. Because the header value is bounded only by the deployment's maximum HTTP header size, a single preflight can block request processing for a noticeable amount of time; on runtimes that share one execution thread across requests, this stalls concurrent requests as well. No authentication, special origin, or user interaction is required. This issue arises for any application using "cors()" with the default (or an empty) "allowHeaders". Applications that set a non-empty "allowHeaders" do not reach the affected path. Impact An unauthenticated attacker can send preflight requests that each consume disproportionate CPU relative to their size, degrading or denying service. This is a denial-of-service issue only; it does not expose or modify data.