CVE-2026-126073CVE-2026-126073

Action Pack is a framework for handling and responding to web requests. Prior to 8.1.4.1 and 8.0.5.2, Action Pack negotiates the format of a rendered response from the Accept, X-Requested-With and, for XHR requests without Accept, Content-Type request headers, but ActionController::Rendering#_set_vary_header adds Vary: Accept only when the request carries a usable Accept header and the response has no Vary header yet. A response to a request without Accept, or to an XHR request, therefore omits the headers that selected its representation, and a response whose Vary the application set before rendering (for example Vary: Accept-Language) never gains Accept. A shared cache that honours Vary could store such a response and serve it to later clients that request a different representation, possibly leading to cache poisoning in which visitors receive, until the entry expires, a page without its layout or HTML in place of JSON. The attacker cannot choose the content of the response. Applica