Write up CVE-2026-41316 (#3916)

Author: k0kubun

en/news/_posts/2026-04-21-erb-cve-2026-41316.md

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+---
+layout: news_post
+title: "CVE-2026-41316: ERB @_init deserialization guard bypass via def_module / def_method / def_class"
+author: "k0kubun"
+translator:
+date: 2026-04-21 07:51:00 +0000
+tags: security
+lang: en
+---
+
+We published security advisory for CVE-2026-41316.
+
+## CVE-2026-41316: ERB @\_init deserialization guard bypass via def\_module / def\_method / def\_class
+
+A deserialization vulnerability exists in ERB. This vulnerability has been assigned the CVE identifier [CVE-2026-41316](https://www.cve.org/CVERecord?id=CVE-2026-41316). We recommend upgrading the erb gem.
+
+### Scope
+
+Any Ruby application that calls `Marshal.load` on untrusted data AND has both `erb` and `activesupport` loaded is vulnerable to arbitrary code execution. This includes:
+
+- **Ruby on Rails applications that import untrusted serialized data** -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
+- **Ruby tools that import untrusted serialized data** -- any tool using `Marshal.load` for caching, data import, or IPC
+- **Legacy Rails apps** (pre-7.0) that still use Marshal for cookie session serialization
+
+### Details
+
+ERB implements an `@_init` guard to prevent code execution when ERB objects are reconstructed via `Marshal.load` on untrusted data. However, `ERB#def_method`, `ERB#def_module`, and `ERB#def_class` evaluate the template source without checking this guard, allowing an attacker who controls the data passed to `Marshal.load` to bypass the protection and execute arbitrary code. In particular, `def_module` takes no arguments, making it straightforward to invoke as part of a deserialization gadget chain.
+
+Please update the erb gem to version 4.0.3.1, 4.0.4.1, 6.0.1.1, 6.0.4 or later.
+
+### Affected versions
+
+* erb gem 6.0.3 or lower
+
+### Credits
+
+Thanks to [TristanInSec](https://github.com/TristanInSec) for discovering this issue.
+
+## History
+
+* Originally published at 2026-04-21 07:51:00 (UTC)