CVE-2026-126073

Action Pack: Possible cache poisoning in Action Pack content negotiation

Products / packages rails · actionpack

Published 10/11/2026, 20:46:43 UTC

Severity and exploitation

Not in CISA KEVSource: MODERATE

KEV coverage does not rule out exploitation.

KEV checked 14/11/2026, 23:35:33 UTC

OSV · CVSS:3.0 · 4.8

Assessment vector

CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L

GitHub_M · CVSS:3.1 · 4.8

Assessment vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L

The headline uses the highest imported catalog score. Assessments can differ by source and CVSS version.

Severity

medium4.8

CVSS measures technical severity, not the likelihood of an attack.

Known exploitation

Not listed in the checked CISA KEV catalog

Absence from CISA KEV does not rule out attacks.

Public exploit

Not established by this record

Missing evidence does not mean no public exploit exists.

Remediation

Package fixes recorded

Fix versions are recorded for 1 of 1 mapped packages. Check each release branch and any separate product guidance.

What this means

Action Pack in Rails is affected by a possible cache poisoning vulnerability when using a shared cache that honors Vary headers. Versions 8.1 to 8.1.4 and less than 8.0.5.2 are affected. An attacker cannot choose the content of the response.

Summarized by @cf/meta/llama-3.3-70b-instruct-fp8-fast. Verify details against the original sources.

Attack conditions

No attack-condition excerpt has been identified here. Review the full source description and references for authentication, network access, and configuration requirements.

Review supporting sources →

Affected versions

Check the exact model, release, and any platform requirements against your installation.

rails · GitHub_M

rails

2 ranges

GitHub_M version evidence

Check a product version

Compare with this publisher’s ranges. This does not inspect your installation.

affected

>= 8.1, < 8.1.4.1

< 8.0.5.2

Other versions: unknown

Packages

Compare each installed version with the publisher’s affected ranges. Where a fix is recorded, choose the appropriate release branch and test compatibility before upgrading.

actionpack RubyGems

Affected range	Recorded fix
From 8.1 before 8.1.4.1Ecosystem versions	8.1.4.1
Earlier releases before 8.0.5.2Ecosystem versions	8.0.5.2

Version evidence: OSV · GHSA-r9jg-fmfx-2gv6

View original version evidence (518 explicit versions)

What to do

Compare your installed product and version with the affected versions above. A matching product name alone does not confirm exposure.

Use the recorded fix for your release branch. Review the publisher’s instructions and test compatibility before upgrading.

cve.new guidance unless attributed to a publisher. This page does not inspect your installation.

Public exploit evidence

No public exploit reference is identified in the imported evidence. This is not a claim that no exploit exists.

Original source description

Read the full publisher description

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:

Credit

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

References

7 sources

rails/rails · security/advisories/GHSA-r9jg-fmfx-2gv6

Security advisoryGitHub_M

rails/rails · commit 7a955ca

PatchGitHub_M

rails/rails · commit b58c107

PatchGitHub_M

rails/rails · repository

Source reference

rails/rails · releases/tag/v8.0.5.2

Release notesGitHub_M

rails/rails · releases/tag/v8.1.4.1

Release notesGitHub_M

View 1 more references

rubysec/ruby-advisory-db · GHSA-r9jg-fmfx-2gv6.yml

Source reference

Inside the record

Evidence available

Package identity

Version evidence

Remediation guidance · not supplied

Weakness classification

Additional publishers

Describes the evidence in this view, not whether the product is safe.

Original records

CVE-2026-126073

View the latest unmodified CVE JSON from the official source. It may be newer than this imported page.

cve.new JSON →

Underlying weaknesses

The source’s classification of what caused the vulnerability.

CWE-436 ↗

CWE-436: Interpretation Conflict

GitHub_M

Contributing organizations · 2

GitHub_M

Assigning organization

2026-11-10

CISA-ADP

Additional data publisher

2026-11-12

Timeline

All times UTC

10 Nov 2026, 20:46
Advisory published
Publication date supplied by the source.
12 Nov 2026, 18:01
Source updated
Latest modification date supplied by the source.
12 Nov 2026, 18:30
Last imported by cve.new
Latest catalog import, not the date we first saw this advisory.

What changed

Follow changes

CVE · CVE-2026-1260732026-11-12

Observed 2026-11-12T18:30:19.565Z · upstream updated 2026-11-12T18:01:27.975Z

Prioritization updated

before

[]

after

[
  {
    "source": "CISA-ADP",
    "type": "SSVC",
    "role": "CISA Coordinator",
    "timestamp": "2026-11-12T16:26:31.033828Z",
    "decisions": [
      {
        "name": "Exploitation",
        "value": "none"
      },
      {
        "name": "Automatable",
        "value": "no"
      },
      {
        "name": "Technical Impact",
        "value": "partial"
      }
    ]
  }
]

Large field values are marked as truncated previews. Original source snapshots remain retained.

Showing up to 50 most recent observed changes.

Reading the signals

Severity · CVSS: Technical impact and attack conditions. The headline is the highest imported catalog score; sources may disagree.
Observed exploitation · KEV: An absent or unavailable KEV listing does not establish that exploitation has not occurred.
Predicted exploitation · EPSS: No EPSS estimate available in this record.

CISA-ADP · SSVC

CISA Coordinator · 2026-11-12

Exploitation: none
Automatable: no
Technical Impact: partial

SSVC describes the publisher’s decision context. Your organization’s exposure and priorities may differ.

Record identifiers

Aliases: GHSA-r9jg-fmfx-2gv6

Source evidence

Request a correction or dispute

Start with the assigning organization for this record. Include the CVE ID, the disputed information, and supporting evidence. A dispute does not itself reject a record.

Find the responsible CNA →Official dispute and escalation process ↗

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