Description
ERB is a templating system for Ruby. 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`, and `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. ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4 patch the issue.
References10
- github.com/ruby/erb/security/advisories/GHSA-q339-8rmv-2mhv
- access.redhat.com/errata/RHSA-2026:18030
- access.redhat.com/errata/RHSA-2026:18039
- access.redhat.com/errata/RHSA-2026:18065
- access.redhat.com/errata/RHSA-2026:20596
- access.redhat.com/errata/RHSA-2026:20606
- access.redhat.com/errata/RHSA-2026:20614
- access.redhat.com/errata/RHSA-2026:20670
- access.redhat.com/errata/RHSA-2026:26312
- access.redhat.com/errata/RHSA-2026:26655
Metrics
- CVSS 3.1
-
8.1
HIGH
Score source: CNA
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H - EPSS — exploit probability
- 1% percentile: 67 · as of 13 November 2026
- CWE
- CWE-693CWE-502
Could this be exploited in your environment?
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