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Medium, CVSS 4.8amended

Action Pack: Possible cache poisoning in Action Pack content negotiation

CVE-2026-126073GHSA-r9jg-fmfx-2gv6

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (actionpack 8.1.4.1, 8.0.5.2), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • RubyGems actionpack

    Affected
    >= 8.1, < 8.1.4.1< 8.0.5.2
    Fixed in
    8.0.5.2, 8.1.4.1

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
4.8 · CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from osv record GHSA-r9jg-fmfx-2gv6

  2. Revision 1 · created ·

    Published from ghsa record GHSA-r9jg-fmfx-2gv6

Agent brief

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# Patch Next advisory brief: CVE-2026-126073

Advisory: Action Pack: Possible cache poisoning in Action Pack content negotiation
IDs: CVE-2026-126073, GHSA-r9jg-fmfx-2gv6
Record: https://patchnext.com/advisories/cve-2026-126073 (revision 2, source checked 11 Nov 2026, 07:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: actionpack 8.1.4.1, 8.0.5.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems actionpack: affected >= 8.1, < 8.1.4.1; < 8.0.5.2; fixed in 8.0.5.2, 8.1.4.1

## Suggested inspection (read-only)
- RubyGems actionpack: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on actionpack.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-r9jg-fmfx-2gv6 (GitHub-reviewed advisory GHSA-r9jg-fmfx-2gv6 lists affected ranges: RubyGems actionpack >=8.1 =0 <8.0.5.2)
- Verified: https://osv.dev/vulnerability/GHSA-r9jg-fmfx-2gv6 (OSV record GHSA-r9jg-fmfx-2gv6 lists affected ranges: RubyGems actionpack >=8.1 =0 <8.0.5.2 (fixed))

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