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MEDIUM severity

CVE-2026-126072 — actionpack

MEDIUMFix: rails/rails@27ad18e

CVE-2026-126072 is a medium-severity (CVSS 5.9) CWE-833 vulnerability in actionpack. A fix is available for actionpack — see the affected versions and patch details below.

Action Pack: Possible DoS vulnerability in Action Controller Live streaming

Also known asGHSA-rwvh-fw6r-pwvp
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Nov 12, 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 CVEs36th percentile — riskier than 36% 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

ActionController::Live::Buffer#close pushes its end-of-stream marker onto the response's bounded queue while it holds the buffer's monitor. When the client has stopped reading and the queue is full, the push blocks, and Buffer#abort, which runs when the client disconnects or a write times out, waits indefinitely for the same monitor. Each such response could permanently hold a web server thread and its ActionController::Live thread, possibly resulting in a DoS vulnerability through thread exhaustion.

Applications that stream responses with ActionController::Live, including send_stream, are affected. Applications that serve large files through Active Storage proxy mode are exposed to unauthenticated clients.

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-LDFW5N2T).

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
💎RubyGemsactionpack≥ 8.1.0.beta1&&< 8.1.4.18.1.4.1bundle update actionpack --conservative
💎RubyGemsactionpackall versions8.0.5.2bundle update actionpack --conservative

Detection & mitigation playbook

Open-source dependency

  1. 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.

  2. 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-2026-126072 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact `ActionController::Live::Buffer#close` pushes its end-of-stream marker onto the response's bounded queue while it holds the buffer's monitor. When the client has stopped reading and the queue is full, the push blocks, and `Buffer#abort`, which runs when the client disconnects or a write times out, waits indefinitely for the same monitor. Each such response could permanently hold a web server thread and its `ActionController::Live` thread, possibly resulting in a DoS vulnerability through thread exhaustion. Applications that stream responses with `ActionController::Live`, including
O3 Security · Impact-Aware SCA

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CVE-2026-126072: actionpack DoS — Fixed in 8.1.4.1