GHSA-8j4g-w8fx-2239

### 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.