Like many similar frameworks and languages, Struts 2 has a “developer mode” (devmode) offering additional features to aid debugging. Error messages will be more verbose, and the devmode includes an OGNL console. OGNL, the Object-Graph Navigation Language, can interact with Java, but in the end, executing OGNL results in arbitrary code execution. This OGNL console resembles a “web shell” built into devmode.
Article Link: Struts "devmode": Still a problem ten years later? - SANS Internet Storm Center