Hono: ReDoS in CORS middleware via Access-Control-Request-Headers

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.