XWiki Platform Old Core is a core package for XWiki Platform, a generic wiki platform. Starting in versions 11.3.7, 11.0.3, and 12.0RC1, it is possible to exploit a bug in XWikiRights resolution of groups to obtain privilege escalation. More specifically, editing a right with the object editor leads to adding a supplementary empty value to groups which is then resolved as a reference to XWiki.WebHome page.

Adding an XWikiGroup xobject to that page then transforms it to a group, any user put in that group would then obtain the privileges related to the edited right. Note that this security issue is normally mitigated by the fact that XWiki.WebHome (and XWiki space in general) should be protected by default for edit rights. The problem has been patched in XWiki 13.10.4 and 14.2RC1 to not consider anymore empty values in XWikiRights.

It's possible to work around the problem by setting appropriate rights on XWiki.WebHome page to prevent users to edit it.