XWiki Platform is a generic wiki platform. Starting in version 6.0, users with write rights can insert well-formed content that is not handled well by the parser. As a consequence, some pages becomes unusable, including the user index (if the page containing the faulty content is a user page) and the page index.
Note that on the page, the normal UI is completely missing and it is not possible to open the editor directly to revert the change as the stack overflow is already triggered while getting the title of the document. This means that it is quite difficult to remove this content once inserted.
This has been patched in XWiki 13.10.10, 14.4.6, and 14.9-rc-1. A temporary workaround to avoid Stack Overflow errors is to increase the memory allocated to the stack by using the -Xss JVM parameter (e.g., -Xss32m). This should allow the parser to pass and to fix the faulty content. The consequences for other aspects of the system (e.g., performance) are unknown, and this workaround should be only be used as a temporary solution. The workaround does not prevent the issue occurring again with other content. Consequently, it is strongly advised to upgrade to a version where the issue has been patched.