Summary

Netty's CorsHandler provides a shortCircuit() configuration designed to reject unauthorized cross-origin requests immediately, acting as a security control before requests reach the application. However, due to a logical operator error in the origin evaluation process, this protection can be entirely bypassed. An attacker can bypass the short-circuit mechanism by sending a request with an Origin: null header. This failure forwards unauthorized requests to the backend application, bypassing intended access controls.

Details

In io.netty.handler.codec.http.cors.CorsHandler#channelRead, the short-circuit logic relies on the configuration returned by getForOrigin(origin) to determine if an origin is authorized. If getForOrigin returns a configuration object, the short-circuit check (!(origin == null || config != null)) is bypassed, and the request proceeds to the backend.

The vulnerability is located in the getForOrigin method:

            if (corsConfig.isNullOriginAllowed() || NULL_ORIGIN.equals(requestOrigin)) {
                return corsConfig;
            }

If an attacker sends Origin: null, NULL_ORIGIN.equals(requestOrigin) evaluates to true. The method returns the configuration object regardless of whether isNullOriginAllowed() was configured by the developer. The short-circuit is bypassed.

Impact

Applications relying on CorsHandler's short-circuit feature to prevent unauthorized cross-origin requests from reaching their backend logic are completely exposed. The framework fails to enforce the developer's intended access controls, allowing unauthorized requests to be processed.