membership_tool.py in Plone before 4.2.3 and 4.3 before beta 1 allows remote attackers to enumerate user account names via a crafted URL.
https://plone.org/products/plone/security/advisories/20121106/13
https://plone.org/products/plone-hotfix/releases/20121106
https://github.com/plone/Products.CMFPlone/blob/4.2.3/docs/CHANGES.txt