It’s possible to know if a user has or not an account in a wiki related to an email address, and which username(s) is actually tight to that email by forging a request to the Forgot username page. Note that since this page does not have a CSRF check it’s quite easy to perform a lot of those requests.
This issue has been patched in XWiki 12.10.5 and 13.2RC1. Two different patches are provided:
It’s possible to fix the problem without uprading by editing the ForgotUsername page in version below 13.x, to use the following code: https://github.com/xwiki/xwiki-platform/blob/69548c0320cbd772540cf4668743e69f879812cf/xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-ui/src/main/resources/XWiki/ForgotUsername.xml#L39-L123
In version after 13.x it’s also possible to edit manually the forgotusername.vm file, but it’s really encouraged to upgrade the version here.
If you have any questions or comments about this advisory:
github.com/xwiki/xwiki-platform
github.com/xwiki/xwiki-platform/commit/69548c0320cbd772540cf4668743e69f879812cf
github.com/xwiki/xwiki-platform/commit/f0440dfcbba705e03f7565cd88893dde57ca3fa8
github.com/xwiki/xwiki-platform/security/advisories/GHSA-vh5c-jqfg-mhrh
jira.xwiki.org/browse/XWIKI-18384
jira.xwiki.org/browse/XWIKI-18408
nvd.nist.gov/vuln/detail/CVE-2021-32732