It has been discovered that TYPO3 is susceptible to session fixation. If a user authenticates while anonymous session data is present, the session id is not changed. This makes it possible for attackers to generate a valid session id, trick users into using this session id (e.g. by leveraging a different Cross-Site Scripting vulnerability) and then maybe getting access to an authenticated session.
github.com/advisories/GHSA-r9vc-jfmh-6j48
github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2015-07-01-2.yaml
github.com/TYPO3/typo3/commit/4c9aba94a930d56ab374693c9c5cc0458587278a
github.com/TYPO3/typo3/commit/4f6e84bba3c13ea8b2652af1a4c47758aa0705f4
typo3.org/security/advisory/typo3-core-sa-2015-003
typo3.org/teams/security/security-bulletins/typo3-core/typo3-core-sa-2015-003