CVE-2026-126073: Ruby on Rails

Summary

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.

cve.now Index

39Pcve.now Index 39 of 100 · methodology v3 · provisional

Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
Exposure17/25No login or user action needed per the CVSS vector, reachable over the network without a login · Unknown: No advisory context yet
Reach17/20Affects Ruby on Rails (a widely deployed server, runtime, framework or database)
Severity4.8/10CVSS 4.8
No patch0/5No signal

What would change the score

Would score about 84 if added to CISA KEV
Would score about 79 if CISA's SSVC rates exploitation active
Would score about 44 if advisories confirm default setups are exposed

How the score works · Rankings

What to do

Upgrade to a fixed version: 8.1.4.1, 8.0.5.2.

Check your systems with an agent

Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.

Check whether my systems are affected by CVE-2026-126073 (Action Pack: Possible cache poisoning in Action Pack content negotiation), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126073 (entry last updated 2026-11-10):
Affected:
- rails rails: from 8.1, fixed in 8.1.4.1; fixed in 8.0.5.2
- RubyGems package actionpack: from 8.1, fixed in 8.1.4.1
- RubyGems package actionpack: fixed in 8.0.5.2
Fixed versions: 8.1.4.1, 8.0.5.2.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126073 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126073 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (RubyGems): find every lockfile in each project and run check_stack from the cve.now MCP server on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/rails/rails/security/advisories/GHSA-r9jg-fmfx-2gv6
- https://github.com/rails/rails/commit/7a955ca36779938267c93902c47d1f55bab99b4b
- https://github.com/rails/rails/commit/b58c107abe9c1f10c1d6a4941402c60e4dc4742c
- https://github.com/rails/rails/releases/tag/v8.0.5.2
- https://github.com/rails/rails/releases/tag/v8.1.4.1
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126073
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126073.

Timeline

Nov 10, 2026, 8:46 PM UTCCVE published
Nov 10, 2026, 9:26 PM UTCFirst seen by cve.now

Affected versions

rails rails
From 8.1 · fixed in 8.1.4.1; fixed in 8.0.5.2
actionpackRubyGems
From 8.1 · fixed in 8.1.4.1
actionpackRubyGems
fixed in 8.0.5.2

Evidence 7

Vendor advisory · 1

Establishes: Affected and fixed versions.

Advisory
GitHub · rails/rails · Advisory GHSA-r9jg-fmfx-2gv6

CVE record · 2

Establishes: The identifier and description.

Record
CVE List · CVE-2026-126073
Record
NVD · CVE-2026-126073

Corrections

None so far.

Something wrong on this page? Report an error. Every fix is listed here.

Questions about CVE-2026-126073

Is CVE-2026-126073 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data

Which versions are affected by CVE-2026-126073?

rails: from 8.1 before 8.1.4.1; before 8.0.5.2. actionpack (RubyGems): from 8.1 before 8.1.4.1. actionpack (RubyGems): before 8.0.5.2.Source: cve.now feed data

How do I fix CVE-2026-126073?

Upgrade to a fixed version: 8.1.4.1, 8.0.5.2.Source: cve.now feed data

Action Pack: Possible cache poisoning in Action Pack content negotiation