Versions Affected: All current Plone versions. Versions Not Affected: None. Nature of vulnerability: Allows creation of members by anonymous users on sites that have self-registration enabled, allowing bypass of CAPTCHA and similar protections against scripted attacks. The patch can be added to buildouts as Products.PloneHotfix20150910 (available from PyPI) or downloaded from Plone.org. Immediate Measures You Should Take: Disable self-registration until you have applied the patch.
Plone's URL checking infrastructure includes a method for checking if URLs valid and located in the Plone site. By passing HTML into this specially crafted url, XSS can be achieved.