Plone 3.3.0 through 3.3.6, 4.0.0 through 4.0.10, 4.1.0 through 4.1.6, 4.2.0 through 4.2.7, 4.3.0 through 4.3.6, and 5.0rc1 allows remote attackers to add a new member to a Plone site with registration enabled, without acknowledgment of site administrator.
https://plone.org/security/hotfix/20150910/anonymous-is-able-to-create-plone-members
https://github.com/zopefoundation/Products.CMFCore/commit/e1d981bfa14b664317285f0f36498f4be4a23406