CVE-2026-126073 Action Pack: Possible cache poisoning in Action Pack content negotiation

What is CVE-2026-126073?

CVE-2026-126073 is a medium-severity vulnerability in rails, published on 10 Nov 2026.

Official description

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.

Weakness: CWE-436

Which versions are affected by CVE-2026-126073?

2 open source packages ship an affected version, including actionpack.

How do I fix CVE-2026-126073?

Upgrade actionpack to a fixed version for your release line: 8.0.5.2 or 8.1.4.1.

Is CVE-2026-126073 being exploited?

There is no known exploitation of CVE-2026-126073 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2026-126073, GHSA-r9jg-fmfx-2gv6, UBUNTU-CVE-2026-126073.