In versions of mruby up to and including 1.4.0, a use-after-free vulnerability exists in src/io.c::File#initilialize_copy(). An attacker that can cause Ruby code to be run can possibly use this to execute arbitrary code.
https://github.com/mruby/mruby/issues/4001
https://github.com/mruby/mruby/commit/b51b21fc63c9805862322551387d9036f2b63433