From https://www.otrs.com/security-advisory-2017-09-security-update-otrs-framework/, released today:
An attacker who is logged into OTRS as an agent can manipulate form parameters and execute arbitrary shell commands with the permissions of the OTRS or web server user.
Fixed in OTRS 5.0.25: https://www.otrs.com/release-notes-otrs-5s-patch-level-25/