ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-41316: 1.13% estimated exploitation activity in the next 30 days; percentile 66.46 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard: - ERB#defmethod - ERB#defmodule - ERB#defclass An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#defmodule (zero-arg, default parameters) as a code execution sink, bypassing the @init protection entirely. <details The @init Guard In ERB#initialize, the guard is set: In ERB#result and ERB#run, the guard is checked before eval(@src): When an ERB object is reconstructed via Marshal.load, @init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singletonclass cannot be marshaled, the attacker cannot set @init to the correct value, and result/run…

Published by the source

24 Apr 2026, 15:36 UTC

Last modified by the source

9 Jun 2026, 10:35 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-q339-8rmv-2mhv

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# Patch Next advisory brief: CVE-2026-41316

Advisory: ERB has an @_init deserialization guard bypass via def_module / def_method / def_class
IDs: CVE-2026-41316, GHSA-q339-8rmv-2mhv
Record: https://patchnext.com/advisories/cve-2026-41316 (revision 1, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: erb 6.0.4, 6.0.1.1, 4.0.4.1, 4.0.3.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems erb: affected < 4.0.3.1; >= 4.0.4, <= 4.0.4; >= 5.0.0, < 6.0.1.1; >= 6.0.2, < 6.0.4; fixed in 4.0.3.1, 6.0.1.1, 4.0.4.1, 6.0.4

## Suggested inspection (read-only)
- RubyGems erb: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on erb.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-q339-8rmv-2mhv (GitHub-reviewed advisory GHSA-q339-8rmv-2mhv lists affected ranges: RubyGems erb >=0 =4.0.4 =5.0.0 =6.0.2 <6.0.4)

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