Impact

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.

Applications that render negotiated responses from actions marked public (for example with expires_in ..., public: true) behind a shared cache that honours Vary, such as Thruster or Rack::Cache, are affected.

Details

In the fixed releases, a rendered response whose format is not given by the URL lists Accept and X-Requested-With in Vary, and also Content-Type for an XHR request without Accept; when config.action_dispatch.ignore_accept_header is enabled, it lists only X-Requested-With. These fields are appended to an existing Vary value rather than skipped, and Vary: * is left unchanged. Shared caches can store more variants of such responses than before. See the Action Pack CHANGELOG for details.

Workarounds

Set Vary: Accept, X-Requested-With on negotiated responses in the application, for example in an after_action, or do not mark negotiated responses public.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/7a955ca36779938267c93902c47d1f55bab99b4b.patch
• 8.0: https://github.com/rails/rails/commit/b58c107abe9c1f10c1d6a4941402c60e4dc4742c.patch

Credit

This issue was found by the Anthropic OSS Scanner (ANT-2026-7GHGLQGO, ANT-2026-5MT6C2R3).