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CVE-2026-41316 high CVSS 8.1

Vulnerability in erb (CVE-2026-41316)

Summary

Vulnerability in erb (CVE-2026-41316). Successful exploitation can lead to full system takeover. Exploitable via Marshal.load. Mitigation: upgrade to 6.0.4 or later.

Original description

The description as published by the source.

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#def_method
  • ERB#def_module
  • ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

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.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

Source: GitHub Security Advisories (Global) NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-41316 has been found in erb.
Attackers can target a specific entry point like Marshal.load over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 8.1/10.
What to do: upgrade erb to 6.0.4 or later.
If unsure, ask your IT team or search "erb CVE-2026-41316" on the vendor's site.

CVE-2026-41316 (erb) — CWE-693 / CVSS v3 8.1
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Marshal.load / erb / nil / ERB.singleton_class
Patched: 6.0.4 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-693) exists in erb. Attackers reach the vulnerable code path via Marshal.load without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of erb. Attack surface: Marshal.load / erb / nil / ERB.singleton_class.

🔥 Severity

Severity: High (CVSS 8.1/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 6.0.4.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'erb' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'erb' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で erb を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade erb to 6.0.4

    ステージング環境で 6.0.4 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

rubygems erb
[{"type":"ECOSYSTEM","events":[{"introduced":"6.0.2"},{"fixed":"6.0.4"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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