vBulletin 5.5.4 through 5.6.2 allows remote command execution via crafted subWidgets data in an ajax/render/widget_tabbedcontainer_tab_panel request. NOTE: this issue exists because of an incomplete fix for CVE-2019-16759.
https://www.tenable.com/blog/zero-day-remote-code-execution-vulnerability-in-vbulletin-disclosed