XWiki is a generic wiki platform. In versions starting from 15.3-rc-1 to before 15.10.14, from 16.0.0-rc-1 to before 16.4.6, and from 16.5.0-rc-1 to before 16.10.0-rc-1, a user who can access pages located in the XWiki space (by default, anyone) can access the page XWiki.Authentication.Administration and (unless an authenticator is set in xwiki.cfg) switch to another installed authenticator. Note that, by default, there is only one authenticator available (Standard XWiki Authenticator). So, if no authenticator extension was installed, its not really possible to do anything for an attacker. Also, in most cases, if an SSO authenticator is installed and utilized (like OIDC or LDAP for example), the worst an attacker can do is break authentication by switching back to the standard authenticator (thats because its impossible to login to a user which does not have a stored password, and thats usually what SSO authenticator produce). This issue has been patched in versions 15.10.14, 16.4.6, and 16.10.0-rc-1.
The product does not perform an authorization check when an actor attempts to access a resource or perform an action.
Assuming a user with a given identity, authorization is the process of determining whether that user can access a given resource, based on the user’s privileges and any permissions or other access-control specifications that apply to the resource. When access control checks are not applied, users are able to access data or perform actions that they should not be allowed to perform. This can lead to a wide range of problems, including information exposures, denial of service, and arbitrary code execution.