CVE-2026-41316: ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

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.