How severe is CVE-2026-126073?

CVE-2026-126073 has a CVSS score of 4.8/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-126073?

CVE-2026-126073 affects the following packages: actionpack (RubyGems), actionpack (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2026-126073?

Update actionpack to 8.1.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-126073 is resolved across your whole dependency graph.

How do I detect CVE-2026-126073 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for actionpack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-126073 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-126073 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-126073 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-126073?

CVE-2026-126073 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 37th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-126073?

CVE-2026-126073 is classified as CWE-436 (CWE-436). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-126073 published, and has it been updated?

CVE-2026-126073 was published on November 10, 2026 and was last updated on November 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎

💎 RubyGems

Not in CISA KEV

MEDIUM severity

CVE-2026-126073 — actionpack

MEDIUMFix: rails/rails@7a955ca

CVE-2026-126073 is a medium-severity (CVSS 4.8) CWE-436 vulnerability in actionpack. A fix is available for actionpack — see the affected versions and patch details below.

Action Pack: Possible cache poisoning in Action Pack content negotiation

Also known asGHSA-r9jg-fmfx-2gv6

Published

Nov 10, 2026

Updated

Nov 11, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs37th percentile — riskier than 37% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

💎actionpack💎actionpack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

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).

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`actionpack`	≥ 8.1&&< 8.1.4.1	8.1.4.1`bundle update actionpack --conservative`
💎RubyGems	`actionpack`	all versions	8.0.5.2`bundle update actionpack --conservative`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for actionpack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update actionpack to 8.1.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-126073 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This vulnerability is rated as Moderate impact because successful exploitation requires specific deployment prerequisites—namely, fronting the application with a shared caching proxy while serving public responses that rely on content negotiation—and does not allow an attacker to inject arbitrary content. Under…

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Source: Red Hat security advisory for CVE-2026-126073 (CC BY 4.0)

Frequently Asked Questions

### 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