Echo: CSRF Protection Bypass via MethodOverride Rewriting POST to Safe Method

Echo is a Go web framework. Prior to 4.16.0 and 5.4.0, Echo's MethodOverride middleware accepts a safe HTTP method as the override target for a POST request. When MethodOverride is registered before CSRF in the post-routing middleware chain, route selection retains the original state-changing POST handler but the override changes the method before CSRF evaluates it, causing both token validation and Sec-Fetch-Site checks to be skipped. A cross-site exploit requires an authenticated victim and an override value that a browser-submitted request can control, such as the form getter used by the advisory reproducer; the default X-HTTP-Method-Override header getter is not settable by a cross-site form. This issue is fixed in 4.16.0 and 5.4.0.