Failing to properly encode user input, login status display is vulnerable to cross-site scripting in the website frontend. A valid user account is needed in order to exploit this vulnerability - either a backend user or a frontend user having the possibility to modify their user profile.

Template patterns that are affected are

• ###FEUSER_[fieldName]### using system extension felogin
• for regular frontend rendering (pattern can be defined individually using TypoScript setting config.USERNAME_substToken)

References

• https://github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms-core/2018-12-11-3.yaml
• https://typo3.org/security/advisory/typo3-core-sa-2018-008