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.