Rails Action Pack prior 8.0.5.2/8.1.4.1 Rendering Rendering#_set_vary_header code injection

Summaryinfo

A vulnerability was found in Rails Action Pack. It has been declared as critical. This affects the function ActionController::Rendering#_set_vary_header of the component Rendering. The manipulation results in code injection. This vulnerability is reported as CVE-2026-126073. The attack can be launched remotely. No exploit exists. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in Rails Action Pack. It has been classified as critical. Affected is the function ActionController::Rendering#_set_vary_header of the component Rendering. The manipulation with an unknown input leads to a code injection vulnerability. CWE is classifying the issue as CWE-94. The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment. This is going to have an impact on integrity, and availability. CVE summarizes:

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. 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. This issue is fixed in versions 8.1.4.1 and 8.0.5.2.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-126073 since 11/10/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1059.

Upgrading to version 8.0.5.2 or 8.1.4.1 eliminates this vulnerability.

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• Rails

Name

• Action Pack

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