XWiki allows remote code execution through the extension sheet

XWiki Platform is a generic wiki platform. Starting in version 3.3-milestone-1 and prior to versions 15.10.9 and 16.3.0, on instances where `Extension Repository Application` is installed, any user can execute any code requiring `programming` rights on the server. This vulnerability has been fixed in XWiki 15.10.9 and 16.3.0. Since `Extension Repository Application` is not mandatory, it can be safely disabled on instances that do not use it as a workaround. It is also possible to manually apply the patches from commit 8659f17d500522bf33595e402391592a35a162e8 to the page `ExtensionCode.ExtensionSheet` and to the page `ExtensionCode.ExtensionAuthorsDisplayer`.